Journal of Analytical Toxicology 2014;38:368 –374 doi:10.1093/jat/bku034 Advance Access publication April 29, 2014
Article
Variability in Metabolism of Imipramine and Desipramine Using Urinary Excretion Data† Kelley Ramey1, Joseph D. Ma1,2, Brookie M. Best1,3, Rabia S. Atayee1,2 and Candis M. Morello1,4* 1
Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California San Diego (UCSD), Pharmaceutical Sciences Building (PSB), Dean’s Suite, Room 1121, 9500 Gilman Drive, MC 0657, La Jolla, CA 92093-0657, USA, 2Doris A. Howell Palliative Care Service, San Diego, CA, USA, 3UCSD Department of Pediatrics, Rady Children’s Hospital, San Diego, CA, USA and 4Diabetes Intense Medical Management Clinic, Veterans Affairs San Diego Healthcare System, La Jolla, CA, USA *Author to whom correspondence should be addressed. Email: [email protected]
Variability in imipramine and desipramine metabolism was evaluated using urinary excretion data from patients with pain. Liquid chromatography – tandem mass spectrometry was used to quantitate concentrations in urine specimens. Interpatient population contained 600 unique imipramine specimens, whereas intrapatient population had 137 patients with two or more specimens. Normal concentration ranges of imipramine, desipramine and the desipramine/imipramine metabolic ratio (MR) were established, and various factors were tested for MR impact. Geometric mean of imipramine urine concentration was 0.46 mg/g of creatinine, and desipramine was 0.67 mg/g of creatinine. Gender, concomitant known CYP2C19 inhibitor use and urine pH did not affect MR. However, proton-pump inhibitor (PPI) users had a significantly lower mean MR than those without a listed PPI. Early age group (18– 36 years) had a significantly higher mean MR than middle (37 – 66 years) and late (67 – 90 years) age groups. Approximately one-third were positive for one or more of hydrocodone, oxycodone, hydromorphone or oxymorphone. Patients with no opioids reported in the medication list had a significantly lower geometric mean MR than those with prescribed opioids (1.03 vs. 1.54, P 5 0.004). Patients with only one prescribed opioid had a lower MR than those with two or more prescribed opioids. Patients with younger age, prescribed opioids and no listed PPI were more likely to have a higher geometric mean urinary desipramine/ imipramine MR.
Introduction Imipramine is a tricyclic antidepressant (TCA) that can be prescribed for the treatment of neuropathic pain (NP). Although the primary use of TCAs for the treatment of depression has decreased with the introduction of many other antidepressants, which have a more tolerable side-effect profile compared with the TCAs, the TCAs remain an effective therapeutic option for certain types of depressive illnesses and NP. TCAs are considered firstline treatment for NP, and are effective for central poststroke NP, painful polyneuropathy and painful diabetic neuropathy (1, 2). Both norepinephrine and serotonin reuptake inhibition are important in the treatment of NP (2), yet TCAs have also been shown to have direct opioid receptor activity (3). All TCAs have low affinity for the m-opioid receptor, and imipramine and desipramine have higher affinity for the k-opioid receptor over other receptor subtypes (3). Stimulation of these various opioid receptors may be the means through which the TCAs elicit their analgesic effects, which have been documented in both humans and rats (3). Also, stimulation of d-opioid † Presented as a poster at: PAINWeek: National Conference on Pain for Frontline Practitioners, 5– 8 September 2012, Las Vegas, NV.
receptors may help with depression (3). Several studies have found that opioid receptor antagonists block the antinociceptive effects of TCAs (3, 4) and when administered with morphine, TCAs provide synergistic analgesia (3). The effects of TCAs on opioid receptors are not fully understood, but these interactions may be vital to their efficacy in the treatment of NP and depression. Imipramine’s affinity for muscarinic acetylcholine receptors is the cause of adverse effects such as dry mouth, blurry vision, urine retention, constipation, memory loss and tachycardia (5). Strong binding to histamine H1-receptors can cause moderate sedation and lower the seizure threshold (6). These effects are less common with desipramine due to its secondary amine structure (6). Finally, overdoses of both imipramine and desipramine can have cardiotoxic effects, where imipramine can cause QT prolongation and desipramine can induce changes in the QRS complex (6). Understanding imipramine’s metabolism and sources of its variability in and among patients will help clinicians recognize why certain patients are less able to tolerate the drug than others. Imipramine is extensively metabolized in the liver by cytochrome P450 (CYP) enzymes. Imipramine undergoes N-demethylation to its main, active metabolite desipramine via CYP2C19, CYP1A2 and CYP3A4. CYP2C19 contributes the most to this process, while CYP1A2 and CYP3A4 have smaller contributions, with wide fluctuations in CYP2C19 and CYP3A4 activity among poor and extensive metabolizers (7–9). CYP2D6 is responsible for hydroxylation of both imipramine and desipramine into 2-, 10-hydroxyimipramine and 2-, 10-hydroxydesipramine, respectively. Desipramine does not undergo metabolism by any other CYP enzymes and thus is more sensitive to CYP2D6 polymorphisms. For example, the plasma metabolic ratio (MR) of desipramine/imipramine is able to discriminate between CYP2D6 extensive and poor metabolizers (10, 11). This is of clinical significance in that response to desipramine use may differ based on CYP2D6 metabolizer status. Whether the same MR in urine is able to reproduce findings in plasma is unknown. A large majority of imipramine metabolites are recovered in the urine (.75%), while up to 20% is excreted via the bile and feces (12). Imipramine metabolites in the urine are found in the following amounts: imipramine þ desipramine (1–4%), other non-conjugated metabolites (15–35%), glucuronide metabolites (40–60%) and non-extractable polar metabolites (20–30%) (13). In the population of patients with pain, where often one drug is not sufficient to treat the pain, it is essential to understand the potential interactions between imipramine and medications taken concomitantly, especially opioids. In addition, variability in drug response and metabolism is due to genetic, non-genetic and environmental factors. This analysis of urinary excretion data examined factors such as age, sex, urine pH and concomitant
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medications to determine their effect on imipramine and desipramine metabolism.
Methods Urine specimens for patients being treated for chronic pain were analyzed for routine clinical care using LC–MS-MS at Millennium Laboratories (San Diego, CA, USA). For this study, a retrospective data analysis was conducted using de-identified data already collected from clinical specimens to determine the urinary concentrations of imipramine, desipramine and creatinine. The dataset included subject identification number, specimen identification number, date of urine sample collection, subject’s date of birth, sex, physician’s practice code, subject’s current medication list, urine-creatinine concentration, urine pH and urine drug concentrations of imipramine, desipramine and other pain medications. Institutional Review Board-exempt status was granted by the University of California –San Diego Human Research Protection Program. Subjects and specimens Between January 2011 and April 2012, 715,651 urine specimens were tested for imipramine and desipramine. Selected specimens were positive for both imipramine and desipramine, defined as concentrations of 50 ng/mL. Specimens with urine-creatinine concentration of ,20 mg/dL were excluded to limit for subject variability due to hydration status. Patients who met the inclusion criteria with one visit or the first of several visits were defined as the intersubject population. The intrasubject population was defined as subjects who met the inclusion criteria with two or more visits.
LC –MS-MS analysis An Agilent 1200 series binary pump SL LC system, well-plate sampler and thermostatted column compartment paired with an Agilent Triple Quadrupole mass spectrometer and an Agilent Mass Hunter software were used for analysis of imipramine and desipramine. Chromatographic separation was performed using an acetonitrile – formic acid – water gradient running at 0.4 mL/min and a 2.1 50 mm2, 1.8-mm Zorbax SB-C18 column. Mobile phase A ¼ þ0.1% formic acid in water, B ¼ 0.1% formic acid in acetonitrile and column temperature was set to 508C. Samples were prepared for injection by incubating 25 mL of urine with 50 units of b-glucuronidase Type L-II from Patella vulgata (keyhole limpet) Sigma Product number G 8132 (Sigma-Aldrich Corp., St. Louis, MO, USA) in 50 mL of 0.4 M acetate buffer ( pH 4.5) for 3 h at 458C. Five microliters of the solution was injected for each sample. All spectra were collected using positive electrospray ionization. The optimized instrumental parameters were as follows: gas temperature, 3508C; drying gas, 12 L/min; nebulizer gas (nitrogen), 35 psi (24,100 Pa); capillary voltage, 3,000 V and fragmenter voltage, 60 V. Multiple reaction monitoring (MRM) mode was used for quantitation. Scan time was set to 500 ms. In the MRM mode, two transitions were used to identify and quantitate a single compound. Data were acquired running the QQQ in MRM mode, using transitions imipramine: 281.1 ! 86, 281.1 ! 58 and desipramine: 267.1 ! 72, 267.1 ! 44.
A quantitative transition was used to calculate concentration based on the quantifier ion, and a second transition was used to ensure accurate identification of the target compound based on the ratio of the quantifier ion to the quantifier ion. HPLC-grade water, acetonitrile, methanol and formic acid were obtained from VWR (Westchester, PA, USA). Imipramine and desipramine were obtained from Cerrilliant Corp. (Round Rock, TX, USA). The deuterated internal standards were diluted to 1,000 ng/mL by adding them to synthetic urine (Microgenics Corp., Fremont, CA, USA). Quantitative analysis was performed using the Agilent Mass Hunter Quantitative Analysis software. A four-point calibration curve was created by using a linear fit and forcing the line to go through the origin. Accepted accuracy for calibrators was +20% of the target value and the coefficient of determination (R 2) was required to be 0.99 as verification of linearity and goodness of fit. The lower limit of quantitation for both the imipramine and desipramine was 50 ng/mL. The upper limit of linearity for both the imipramine and desipramine assays was 50,000 ng/mL.
Calculations, statistical methods and graphical analyses Imipramine and desipramine concentrations were normalized using creatinine concentration to account for hydration status (14). The analyte concentration (ng/mL) was divided by the urine-creatinine level (mg/dL), and divided by 10 to correct the volume, with the final concentration reported as milligram analyte per gram creatinine (mg/g cr). The MR was defined as the concentration ratio of desipramine to imipramine, a unitless value. Dosage information was not originally collected as part of the routine clinical urine drug monitoring, so doses were not available for this retrospective analysis. In the absence of dosage information, the MR is the best estimate of imipramine metabolism to desipramine. The imipramine and desipramine creatininecorrected urine concentrations were also summed together by converting from milligrams of drug to millimoles. Descriptive statistics and graphical analyses were performed using Microsoft Excelw 2010 (Microsoft Corp., Redmond, WA, USA) and OriginPro 8.6 (OriginLab Corp., Northampton, MA, USA). To achieve normal distributions of analyte concentrations, the creatinine-corrected concentrations were log transformed. Log-transformed data were used to calculate descriptive statistics. The log-transformed data were back-transformed to determine the geometric mean and geometric 95% confidence intervals (CIs).
Concomitant medications Each concomitant drug that was detected in the urine specimen was tested individually for its effect on the log MR. For each case, two-sample populations were created: samples that were positive for an opioid, benzodiazepine or other pain medications versus those that were negative for that concomitant medication. Differences in log MR were then compared between populations of reported and no reported opioid use, using provider-reported medication lists. Differences in log MR were also compared between the population reporting two or more opioids with the population reporting only one opioid. Similarly, the effect of CYP2C19 inhibitors on the log MR was tested between groups Metabolism of Imipramine and Its Metabolite Desipramine 369
with reported use and no reported use of a CYP2C19 inhibitor. CYP2C19 inhibitors were determined from the ‘P450 Drug Interaction Table’ from the Indiana University’s School of Medicine website (15). For these comparisons above, two-sided, two sample t-tests were performed, and a significance level of 0.05 was used.
Evaluation of other factors Age, sex and urinary pH were also evaluated for their effect on the MR and on the sum of imipramine plus desipramine (I þ D). Based on the age groups from a previous study (16), the interpatient population was divided into three age categories: young (18–36 years), middle (37–66 years) and old (67–90 years). The corresponding log MR and I þ D values for the three age groups were compared with an one-way ANOVA test. A two-sample t-test compared MR and I þ D by sex. Linear regression analyses using the least-squares method was performed using urine pH; values ranged from 4.6 to 9.3.
Results Of the 715,651 patients tested for imipramine, 882 tested positive for imipramine in the urine and satisfied our inclusion criteria. After removing duplicate specimens from the same patient ID, 600 unique specimens remained, characterizing the interpatient population. Of the 600 patients, only 105 had imipramine reported in the medications list. The intrapatient population consisted of 137 patients with two or more positive imipramine urine specimens from separate visits. The number of visits in the intrapatient population ranged from 3 to 13 visits. Imipramine and desipramine concentrations did not exceed the upper limit of linearity (50,000 ng/mL). No subjects reported desipramine in the medication list. Imipramine and desipramine concentrations are shown in Figure 1. A Gaussian distribution was observed for the logtransformed concentrations of imipramine, desipramine and MR. The imipramine geometric mean was 0.461 mg/g cr and the 95% CI of the mean was 0.417 –0.508. The geometric mean for desipramine concentration was 0.674 mg/g cr and the 95% CI of the mean was 0.606 – 0.750. The observed range of desipramine concentrations was slightly higher than imipramine. The geometric mean for the sum of imipramine plus desipramine was 4.605 nmoles/g cr and the 95% CI of the mean was 4.181 – 5.072. For the intrapatient population, the imipramine geometric mean urine concentration was 0.600 mg/g cr and the 95% CI of the mean was 0.502 – 0.716. The desipramine geometric mean concentration was 0.810 mg/g cr and the 95% CI of the mean was 0.665 –0.987. Once again, urine desipramine concentrations were slightly higher than imipramine concentrations. For the intrasubject population, the geometric mean for the sum of imipramine plus desipramine was 6.219 nmoles/g cr and the 95% CI of the mean was 4.981–7.764. Linear regression analysis was used to determine the relationship between imipramine and desipramine concentrations, as well as imipramine concentration and MR. A positive correlation (y ¼ 0.79x þ 0.098, R 2 ¼ 0.52; Figure 2) was observed between imipramine and desipramine concentrations. 370 Ramey et al.
The relationship between parent drug and metabolite was further assessed by examining MR. The MRs were similar between the two populations; the geometric mean for the interpatient population was 1.47 with a 95% CI of 1.36 – 1.59, whereas the intrapatient geometric mean was 1.35 with a 95% CI of 1.17 – 1.57. Linear regression with imipramine concentration and MR produced a poor linear fit (R 2 ¼ 0.068; Figure 1d), showing that a correlation between the two was quite small.
Other factors and MR variability The young age group had a significantly higher mean log MR than the middle and older age groups, as shown in Table I, with geometric means of 1.95, 1.45 and 1.22, respectively. Sex, CYP2C19 inhibitors and urine pH were not significantly related to the imipramine MR; all relevant two-sample t-tests produced results of P . 0.05. However, proton-pump inhibitors (PPIs), when tested alone without the other CYP2C19 inhibitors, were significantly associated with lower imipramine MR (1.12 vs. 1.50, P ¼ 0.02). The patients had the following PPIs in their medication list: omeprazole (20), esomeprazole (17), pantoprazole (4), rabeprazole (3) and lansoprazole (1). The young group also had a significantly lower sum of I þ D in the urine compared with the middle age group, with geometric means in the young, middle and older age groups of 3.24, 4.98 and 4.35, respectively (P ¼ 0.018). When assessing the sum of I þ D, females had higher values than males, 5.06 + 3.26 vs. 3.81 + 3.43 nmoles/g cr (P ¼ 0.0062). Urine pH showed a significant negative relationship with I þ D, but the correlation was weak (R 2 ¼ 0.013).
Concomitant medications About 30% of patients were positive for one or more of the following: hydrocodone, oxycodone, hydromorphone or oxymorphone. Fourteen percent were positive for morphine, and 9% were positive for methadone. The concomitant opioid frequencies are shown in Table II. Evaluation of the individual opioids for their impact on the imipramine MR produced no significant results. However, imipramine patients with no reported opioids in the medication list had a significantly lower geometric mean MR than the opioid-reporting population, as shown in Table II. Furthermore, patients with two or more reported opioids had a significantly higher MR than those with only one reported opioid (1.71 vs. 1.45, P , 0.05). A slightly, but not significantly, higher mean MR was found in subjects positive for oxymorphone (P ¼ 0.07), with geometric mean MRs of 1.59 and 1.40, respectively. A slightly higher MR was also seen for patients positive for hydromorphone (P ¼ 0.09), where the respective means were 1.57 and 1.41. Other common concomitant drugs, including gabapentin, carisoprodol and benzodiazepines, were not significantly related to the MR (P . 0.05).
Discussion The results of this study help establish observed normal ranges for urinary concentrations of imipramine and desipramine. The mean desipramine urine concentration was slightly higher than
Figure 1. (a) Log creatinine-corrected imipramine concentrations. The median (interquartile range) imipramine urine concentrations were 0.484 mg/g creatinine (0.187–1.11). (b) Log creatinine-corrected desipramine concentrations. The median (interquartile range) desipramine urine concentrations were 0.712 mg/g creatinine (0.252–1.82). (c) Log creatininecorrected MRs. The median (interquartile range) urine MRs were 1.43 (0.754– 2.77). (d) Relationship between imipramine concentration and MR. The MR showed a weak negative relationship with the imipramine urine concentration (y ¼ 20.21x ¼ 0.098, R 2 ¼ 0.068, P , 0.0001). GM, geometric mean.
the mean imipramine urine concentration. Desipramine plasma half-life has a wide reported range of 7 – 60 h, and imipramine plasma half-life is 8 – 16 h (6). The wider range of desipramine plasma half-life may be due to the variability in hepatic metabolism (6). Imipramine also undergoes hepatic metabolism; but has multiple CYP enzymes involved, leading to a wide bioavailability range after oral administration of 22 –77%. Enterohepatic circulation may occur for both drugs. Elimination rates from the plasma will include metabolism and urinary excretion, whereas elimination rates in the urine represent only the fraction of the drug and drug metabolites that get excreted via the urine. Less than 5% of imipramine is excreted unchanged in the urine (6) and desipramine fraction excreted unchanged is ,3%, although this varies between different ethnic groups (17). The scatterplot in Figure 2 shows a positive relationship between desipramine and imipramine urine concentrations. The parent drug and metabolite were moderately positively correlated, with an R 2-value of 0.52. Besides demethylation to desipramine by
CYP2C19, CYP1A2 and CYP3A4, imipramine undergoes hydroxylation by CYP2D6 to 2- and 10-hydroxyimipramine. No evidence of saturation of metabolism was noted in this study. This is not surprising for two main reasons. First, imipramine is metabolized by multiple CYP enzymes and secondly desipramine concentrations, not imipramine concentrations, have been found to increase with an increasing imipramine dose (18).
Age The young age group had a significantly higher mean log MR than the middle and older age groups, which agrees with previous studies on concentrations in plasma. Studies in children have reported higher concentrations of TCA metabolites compared with adults (19 – 21). The young also had the lowest total amount of active drug in urine, imipramine plus desipramine. Steady-state plasma concentrations of imipramine were found to be higher in an older population, ranging from 50 to 65 year Metabolism of Imipramine and Its Metabolite Desipramine 371
Figure 2. Relationship between imipramine and desipramine. Urine desipramine is positively correlated with urine imipramine concentrations, but the slope of 0.79 shows that they are not directly proportional (y ¼ 0.79x þ 0.098, R 2 ¼ 0.52, P , 0.0001).
Table I Effect of Various Factors on MR
Age Young (Y, 18– 36 years) Middle (M, 37–66 years) Old (O, 67–90 years) Gender Male Female CYP2C19 inhibition Inhibition No inhibition PPIs PPI prescribed No PPI prescribed Opioids Opioids prescribed No opioids prescribed 2þ opioids Two or more opioids prescribed One opioid prescribed
N
Geometric mean MR
Significance
70 417 97
1.95 1.45 1.22
Y–M: P ¼ 0.047; Y–O: P ¼ 0.005; M–O: P ¼ 0.241
201 397
1.38 1.51
P ¼ 0.125
71 529
1.31 1.49
P ¼ 0.145
45 555
1.12 1.50
P ¼ 0.024
476 46
1.54 1.03
P ¼ 0.004
173 303
1.71 1.45
P ¼ 0.039
Table II Type and Frequency of Concomitant Opioids Opioid
# Subjects with drug in the medication list
# Subjects with drug in urinea
% Subjects with drug in the medication list
% Subjects with drug in urinea
Oxycodone Hydrocodone Morphine Fentanyl Tramadol Methadone Oxymorphone Buprenorphine Hydromorphone Codeine Propoxyphene Meperidine Opium
208 200 73 45 34 33 23 18 12 10 4 1 1
186 190 81 41 27 56 196 15 205 10 1 2 N/A
35 33 12 8 6 6 4 3 2 2 0.7 0.2 0.2
31 32 14 7 5 9 33 3 34 2 0.2 3 0
a
Concentration of drug is above the cutoff value.
372 Ramey et al.
old, than a younger population ranging from 30 to 39 year old (22). One explanation for this finding is that younger people have a more rapid demethylation pathway than older people, whereas the hydroxylation pathways are comparable. Possible explanations for this include age-related changes in enzyme function and changes in hepatic blood flow (22). Enzyme function may be decreased for several reasons. Decreased quantities of hepatic smooth endoplasmic reticulum with age may contribute to reduced activity of NADPH-cytochrome P450 reductase, or decreased quantity of cytochrome P450 enzymes, or both (23). The decrease in metabolism appears to affect demethylation of imipramine more than hydroxylation of desipramine, leading to higher concentrations of the parent drug and a lower MR. Studies have also found a decline in imipramine clearance in the elderly with no change in bioavailability, suggesting that this is due to impaired demethylation to desipramine (23). Another study, however, has shown a disproportionate increase in desipramine levels when imipramine dose was increased (18), suggesting that hydroxylation, and not demethylation, was the saturable pathway. While both pathways are variable between patients, the overall consensus on imipramine metabolism is that it is impaired in the elderly, which is consistent with the findings in this study. No significant difference in the MR was found between men and women, which is consistent with previous studies (19, 22). This study did find a higher sum of imipramine plus desipramine in women compared with men. Some studies have found higher TCA concentrations in women, but these findings have been attributed to concomitant medications such as oral contraceptives, which can inhibit hepatic enzymes such as CYP3A (17). The lack of MR difference may also be due to CYP2D6 inhibition, which would result in expected increases in urinary concentrations of both imipramine and desipramine, thus resulting in no change in the overall MR.
Proton-pump inhibitors The PPIs, that is omeprazole, esomeprazole, lansoprazole, pantoprazole and rabeprazole, are known inhibitors of CYP2C19 (13). All PPIs are extensively metabolized in the liver, with the CYP2C19 enzyme being a major metabolic pathway that is in turn inhibited from metabolizing other drugs (24). Lansoprazole and omeprazole are the most potent inhibitors of CYP2C19, while rabeprazole and pantoprazole are the least potent inhibitors (24). Inhibition constants (Ki) for the PPIs are as follows: lansoprazole 0.4–1.5 mM, omeprazole 2–6 mM, esomeprazole 8 mM, pantoprazole 14 – 69 mM and rabeprazole 17 – 21 mM (25). A decrease in MR for patients with a PPI in the medication list can be explained by inhibition of demethylation of imipramine, leading to increased imipramine and thus lower MR. One would expect other CYP2C19 inhibitors to lower MR, but this was not the case in the present study. One possible explanation is that urinary MRs are not robust to detect subtle changes via CYP2C19 inhibition.
Opioids Patients with opioids contained in their medication list had a significantly higher MR than those without concomitant opioids (1.54 vs. 1.03, P ¼ 0.004). While tests assessing each individual
opioid were not significant in their effect on imipramine MR, opioids in general may increase it. It should be noted that inaccuracies in the physician-reported medication list were a possibility, and some lists (78) were not filled out. One possible explanation for this increased MR is the inhibition of desipramine metabolism by CYP2D6. Tramadol, codeine, oxycodone and hydrocodone are all substrates of CYP2D6 (23), and methadone is a known competitive inhibitor of CYP2D6 (26). Also, drugs metabolized by CYP2D6 often follow nonlinear pharmacokinetics due to the enzyme’s saturable nature (27, 28). In addition, genetic polymorphisms of CYP2D6 exist, with .80 known variant alleles. Several CYP2D6 variant alleles are known to result in enzyme activity that may be normal, increased, decreased or even absent (27, 29). The prevalence of these mutations differs among ethnic groups, with 7% of Caucasians and 1 –3% of other ethnic groups known to have limited CYP2D6 function (28). While this is only one possible explanation that was seen as a class effect, there is a need for further research in this matter. Physicians may want to proceed with caution when prescribing opioids concurrently with imipramine. Impairment of metabolism due to either impaired CYP2D6 function or concomitant use of medications that are CYP2D6 substrates or inhibitors may result in increased levels of desipramine. Elevated TCA plasma concentrations lead to increased adverse effects and can lead to serious events such as prolonged cardiac QT interval, central nervous system effects, coma and even death (30, 31). The decreased availability of CYP2D6 may also cause a slight increase in imipramine plasma concentration, as this is also a substrate of the enzyme. Patients taking the aforementioned opioids may need a lower imipramine dose to prevent increased plasma concentrations of both the parent drug and the metabolite.
While individual opioids alone were not significantly associated with the MR, patients positive for oxymorphone or hydromorphone had a slightly increased MR. Patients with no opioids in their medications list had a significantly lower MR than those with prescribed opioids. Furthermore, patients with only one prescribed opioid had a lower MR than those with two or more prescribed opioids. This suggests that opioids may inhibit CYP2D6 metabolism of desipramine or induce N-demethylation of imipramine, leading to higher concentrations of this active metabolite. This analysis of urinary excretion ranges of imipramine and desipramine establishes typical urinary concentrations for pain patients and discusses variables that can alter those ranges. Physicians may want to closely monitor imipramine doses in younger patients, female patients and patients taking concomitant opioids or PPIs. Adverse effects have been shown to occur over very wide ranges of both imipramine and desipramine plasma concentrations, but the ranges listed here may be useful to clinical providers as a reference.
Limitations A key limitation of this study is that dose amount and time of dose administration were not known. Changes in MR over time could be due to differences in postdose sampling times and differences in half-lives of the two compounds. Additional limitations include the lack of CYP 2D6 genotype information, which could help interpret MR findings. Finally, the lack of plasma concentration data precludes any direct comparisons between urine findings from this study and expected plasma concentrations. Although these limitations must be taken into consideration when interpreting these results, advantages of this study were a large sample size and robust quantitative drug and metabolite assay results within each subject.
References
Conclusion Urinary concentrations of imipramine and desipramine display large variability both in and among patients. The MR (desipramine/imipramine) was increased in young patients (18–36 year old) compared with both middle (37 – 66 year old) and older (67–90 year old) populations. At the same time, the sum of imipramine plus desipramine in urine was significantly lower for young patients. Sex and CYP2C19 inhibitors were not significantly related to the MR, but PPIs alone were associated with lower MR, possibly due to decreased imipramine metabolism. Females had significantly higher sums of imipramine plus desipramine in urine.
Acknowledgments K.R. was awarded a student fellowship stipend to participate in this project, supported by the UC San Diego Skaggs School of Pharmacy and Pharmaceutical Sciences and an unrestricted gift to the UC San Diego Skaggs School of Pharmacy and Pharmaceutical Sciences from Millennium Research Institute. The authors thank Dr Amadeo J. Pesce for his expert advice and guidance in this project.
Conflict of interest J.D.M. is a paid consultant for Millennium Laboratories.
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Metabolism of Imipramine and Its Metabolite Desipramine 373
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Article
Variability in Metabolism of Imipramine and Desipramine Using Urinary Excretion Data† Kelley Ramey1, Joseph D. Ma1,2, Brookie M. Best1,3, Rabia S. Atayee1,2 and Candis M. Morello1,4* 1
Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California San Diego (UCSD), Pharmaceutical Sciences Building (PSB), Dean’s Suite, Room 1121, 9500 Gilman Drive, MC 0657, La Jolla, CA 92093-0657, USA, 2Doris A. Howell Palliative Care Service, San Diego, CA, USA, 3UCSD Department of Pediatrics, Rady Children’s Hospital, San Diego, CA, USA and 4Diabetes Intense Medical Management Clinic, Veterans Affairs San Diego Healthcare System, La Jolla, CA, USA *Author to whom correspondence should be addressed. Email: [email protected]
Variability in imipramine and desipramine metabolism was evaluated using urinary excretion data from patients with pain. Liquid chromatography – tandem mass spectrometry was used to quantitate concentrations in urine specimens. Interpatient population contained 600 unique imipramine specimens, whereas intrapatient population had 137 patients with two or more specimens. Normal concentration ranges of imipramine, desipramine and the desipramine/imipramine metabolic ratio (MR) were established, and various factors were tested for MR impact. Geometric mean of imipramine urine concentration was 0.46 mg/g of creatinine, and desipramine was 0.67 mg/g of creatinine. Gender, concomitant known CYP2C19 inhibitor use and urine pH did not affect MR. However, proton-pump inhibitor (PPI) users had a significantly lower mean MR than those without a listed PPI. Early age group (18– 36 years) had a significantly higher mean MR than middle (37 – 66 years) and late (67 – 90 years) age groups. Approximately one-third were positive for one or more of hydrocodone, oxycodone, hydromorphone or oxymorphone. Patients with no opioids reported in the medication list had a significantly lower geometric mean MR than those with prescribed opioids (1.03 vs. 1.54, P 5 0.004). Patients with only one prescribed opioid had a lower MR than those with two or more prescribed opioids. Patients with younger age, prescribed opioids and no listed PPI were more likely to have a higher geometric mean urinary desipramine/ imipramine MR.
Introduction Imipramine is a tricyclic antidepressant (TCA) that can be prescribed for the treatment of neuropathic pain (NP). Although the primary use of TCAs for the treatment of depression has decreased with the introduction of many other antidepressants, which have a more tolerable side-effect profile compared with the TCAs, the TCAs remain an effective therapeutic option for certain types of depressive illnesses and NP. TCAs are considered firstline treatment for NP, and are effective for central poststroke NP, painful polyneuropathy and painful diabetic neuropathy (1, 2). Both norepinephrine and serotonin reuptake inhibition are important in the treatment of NP (2), yet TCAs have also been shown to have direct opioid receptor activity (3). All TCAs have low affinity for the m-opioid receptor, and imipramine and desipramine have higher affinity for the k-opioid receptor over other receptor subtypes (3). Stimulation of these various opioid receptors may be the means through which the TCAs elicit their analgesic effects, which have been documented in both humans and rats (3). Also, stimulation of d-opioid † Presented as a poster at: PAINWeek: National Conference on Pain for Frontline Practitioners, 5– 8 September 2012, Las Vegas, NV.
receptors may help with depression (3). Several studies have found that opioid receptor antagonists block the antinociceptive effects of TCAs (3, 4) and when administered with morphine, TCAs provide synergistic analgesia (3). The effects of TCAs on opioid receptors are not fully understood, but these interactions may be vital to their efficacy in the treatment of NP and depression. Imipramine’s affinity for muscarinic acetylcholine receptors is the cause of adverse effects such as dry mouth, blurry vision, urine retention, constipation, memory loss and tachycardia (5). Strong binding to histamine H1-receptors can cause moderate sedation and lower the seizure threshold (6). These effects are less common with desipramine due to its secondary amine structure (6). Finally, overdoses of both imipramine and desipramine can have cardiotoxic effects, where imipramine can cause QT prolongation and desipramine can induce changes in the QRS complex (6). Understanding imipramine’s metabolism and sources of its variability in and among patients will help clinicians recognize why certain patients are less able to tolerate the drug than others. Imipramine is extensively metabolized in the liver by cytochrome P450 (CYP) enzymes. Imipramine undergoes N-demethylation to its main, active metabolite desipramine via CYP2C19, CYP1A2 and CYP3A4. CYP2C19 contributes the most to this process, while CYP1A2 and CYP3A4 have smaller contributions, with wide fluctuations in CYP2C19 and CYP3A4 activity among poor and extensive metabolizers (7–9). CYP2D6 is responsible for hydroxylation of both imipramine and desipramine into 2-, 10-hydroxyimipramine and 2-, 10-hydroxydesipramine, respectively. Desipramine does not undergo metabolism by any other CYP enzymes and thus is more sensitive to CYP2D6 polymorphisms. For example, the plasma metabolic ratio (MR) of desipramine/imipramine is able to discriminate between CYP2D6 extensive and poor metabolizers (10, 11). This is of clinical significance in that response to desipramine use may differ based on CYP2D6 metabolizer status. Whether the same MR in urine is able to reproduce findings in plasma is unknown. A large majority of imipramine metabolites are recovered in the urine (.75%), while up to 20% is excreted via the bile and feces (12). Imipramine metabolites in the urine are found in the following amounts: imipramine þ desipramine (1–4%), other non-conjugated metabolites (15–35%), glucuronide metabolites (40–60%) and non-extractable polar metabolites (20–30%) (13). In the population of patients with pain, where often one drug is not sufficient to treat the pain, it is essential to understand the potential interactions between imipramine and medications taken concomitantly, especially opioids. In addition, variability in drug response and metabolism is due to genetic, non-genetic and environmental factors. This analysis of urinary excretion data examined factors such as age, sex, urine pH and concomitant
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medications to determine their effect on imipramine and desipramine metabolism.
Methods Urine specimens for patients being treated for chronic pain were analyzed for routine clinical care using LC–MS-MS at Millennium Laboratories (San Diego, CA, USA). For this study, a retrospective data analysis was conducted using de-identified data already collected from clinical specimens to determine the urinary concentrations of imipramine, desipramine and creatinine. The dataset included subject identification number, specimen identification number, date of urine sample collection, subject’s date of birth, sex, physician’s practice code, subject’s current medication list, urine-creatinine concentration, urine pH and urine drug concentrations of imipramine, desipramine and other pain medications. Institutional Review Board-exempt status was granted by the University of California –San Diego Human Research Protection Program. Subjects and specimens Between January 2011 and April 2012, 715,651 urine specimens were tested for imipramine and desipramine. Selected specimens were positive for both imipramine and desipramine, defined as concentrations of 50 ng/mL. Specimens with urine-creatinine concentration of ,20 mg/dL were excluded to limit for subject variability due to hydration status. Patients who met the inclusion criteria with one visit or the first of several visits were defined as the intersubject population. The intrasubject population was defined as subjects who met the inclusion criteria with two or more visits.
LC –MS-MS analysis An Agilent 1200 series binary pump SL LC system, well-plate sampler and thermostatted column compartment paired with an Agilent Triple Quadrupole mass spectrometer and an Agilent Mass Hunter software were used for analysis of imipramine and desipramine. Chromatographic separation was performed using an acetonitrile – formic acid – water gradient running at 0.4 mL/min and a 2.1 50 mm2, 1.8-mm Zorbax SB-C18 column. Mobile phase A ¼ þ0.1% formic acid in water, B ¼ 0.1% formic acid in acetonitrile and column temperature was set to 508C. Samples were prepared for injection by incubating 25 mL of urine with 50 units of b-glucuronidase Type L-II from Patella vulgata (keyhole limpet) Sigma Product number G 8132 (Sigma-Aldrich Corp., St. Louis, MO, USA) in 50 mL of 0.4 M acetate buffer ( pH 4.5) for 3 h at 458C. Five microliters of the solution was injected for each sample. All spectra were collected using positive electrospray ionization. The optimized instrumental parameters were as follows: gas temperature, 3508C; drying gas, 12 L/min; nebulizer gas (nitrogen), 35 psi (24,100 Pa); capillary voltage, 3,000 V and fragmenter voltage, 60 V. Multiple reaction monitoring (MRM) mode was used for quantitation. Scan time was set to 500 ms. In the MRM mode, two transitions were used to identify and quantitate a single compound. Data were acquired running the QQQ in MRM mode, using transitions imipramine: 281.1 ! 86, 281.1 ! 58 and desipramine: 267.1 ! 72, 267.1 ! 44.
A quantitative transition was used to calculate concentration based on the quantifier ion, and a second transition was used to ensure accurate identification of the target compound based on the ratio of the quantifier ion to the quantifier ion. HPLC-grade water, acetonitrile, methanol and formic acid were obtained from VWR (Westchester, PA, USA). Imipramine and desipramine were obtained from Cerrilliant Corp. (Round Rock, TX, USA). The deuterated internal standards were diluted to 1,000 ng/mL by adding them to synthetic urine (Microgenics Corp., Fremont, CA, USA). Quantitative analysis was performed using the Agilent Mass Hunter Quantitative Analysis software. A four-point calibration curve was created by using a linear fit and forcing the line to go through the origin. Accepted accuracy for calibrators was +20% of the target value and the coefficient of determination (R 2) was required to be 0.99 as verification of linearity and goodness of fit. The lower limit of quantitation for both the imipramine and desipramine was 50 ng/mL. The upper limit of linearity for both the imipramine and desipramine assays was 50,000 ng/mL.
Calculations, statistical methods and graphical analyses Imipramine and desipramine concentrations were normalized using creatinine concentration to account for hydration status (14). The analyte concentration (ng/mL) was divided by the urine-creatinine level (mg/dL), and divided by 10 to correct the volume, with the final concentration reported as milligram analyte per gram creatinine (mg/g cr). The MR was defined as the concentration ratio of desipramine to imipramine, a unitless value. Dosage information was not originally collected as part of the routine clinical urine drug monitoring, so doses were not available for this retrospective analysis. In the absence of dosage information, the MR is the best estimate of imipramine metabolism to desipramine. The imipramine and desipramine creatininecorrected urine concentrations were also summed together by converting from milligrams of drug to millimoles. Descriptive statistics and graphical analyses were performed using Microsoft Excelw 2010 (Microsoft Corp., Redmond, WA, USA) and OriginPro 8.6 (OriginLab Corp., Northampton, MA, USA). To achieve normal distributions of analyte concentrations, the creatinine-corrected concentrations were log transformed. Log-transformed data were used to calculate descriptive statistics. The log-transformed data were back-transformed to determine the geometric mean and geometric 95% confidence intervals (CIs).
Concomitant medications Each concomitant drug that was detected in the urine specimen was tested individually for its effect on the log MR. For each case, two-sample populations were created: samples that were positive for an opioid, benzodiazepine or other pain medications versus those that were negative for that concomitant medication. Differences in log MR were then compared between populations of reported and no reported opioid use, using provider-reported medication lists. Differences in log MR were also compared between the population reporting two or more opioids with the population reporting only one opioid. Similarly, the effect of CYP2C19 inhibitors on the log MR was tested between groups Metabolism of Imipramine and Its Metabolite Desipramine 369
with reported use and no reported use of a CYP2C19 inhibitor. CYP2C19 inhibitors were determined from the ‘P450 Drug Interaction Table’ from the Indiana University’s School of Medicine website (15). For these comparisons above, two-sided, two sample t-tests were performed, and a significance level of 0.05 was used.
Evaluation of other factors Age, sex and urinary pH were also evaluated for their effect on the MR and on the sum of imipramine plus desipramine (I þ D). Based on the age groups from a previous study (16), the interpatient population was divided into three age categories: young (18–36 years), middle (37–66 years) and old (67–90 years). The corresponding log MR and I þ D values for the three age groups were compared with an one-way ANOVA test. A two-sample t-test compared MR and I þ D by sex. Linear regression analyses using the least-squares method was performed using urine pH; values ranged from 4.6 to 9.3.
Results Of the 715,651 patients tested for imipramine, 882 tested positive for imipramine in the urine and satisfied our inclusion criteria. After removing duplicate specimens from the same patient ID, 600 unique specimens remained, characterizing the interpatient population. Of the 600 patients, only 105 had imipramine reported in the medications list. The intrapatient population consisted of 137 patients with two or more positive imipramine urine specimens from separate visits. The number of visits in the intrapatient population ranged from 3 to 13 visits. Imipramine and desipramine concentrations did not exceed the upper limit of linearity (50,000 ng/mL). No subjects reported desipramine in the medication list. Imipramine and desipramine concentrations are shown in Figure 1. A Gaussian distribution was observed for the logtransformed concentrations of imipramine, desipramine and MR. The imipramine geometric mean was 0.461 mg/g cr and the 95% CI of the mean was 0.417 –0.508. The geometric mean for desipramine concentration was 0.674 mg/g cr and the 95% CI of the mean was 0.606 – 0.750. The observed range of desipramine concentrations was slightly higher than imipramine. The geometric mean for the sum of imipramine plus desipramine was 4.605 nmoles/g cr and the 95% CI of the mean was 4.181 – 5.072. For the intrapatient population, the imipramine geometric mean urine concentration was 0.600 mg/g cr and the 95% CI of the mean was 0.502 – 0.716. The desipramine geometric mean concentration was 0.810 mg/g cr and the 95% CI of the mean was 0.665 –0.987. Once again, urine desipramine concentrations were slightly higher than imipramine concentrations. For the intrasubject population, the geometric mean for the sum of imipramine plus desipramine was 6.219 nmoles/g cr and the 95% CI of the mean was 4.981–7.764. Linear regression analysis was used to determine the relationship between imipramine and desipramine concentrations, as well as imipramine concentration and MR. A positive correlation (y ¼ 0.79x þ 0.098, R 2 ¼ 0.52; Figure 2) was observed between imipramine and desipramine concentrations. 370 Ramey et al.
The relationship between parent drug and metabolite was further assessed by examining MR. The MRs were similar between the two populations; the geometric mean for the interpatient population was 1.47 with a 95% CI of 1.36 – 1.59, whereas the intrapatient geometric mean was 1.35 with a 95% CI of 1.17 – 1.57. Linear regression with imipramine concentration and MR produced a poor linear fit (R 2 ¼ 0.068; Figure 1d), showing that a correlation between the two was quite small.
Other factors and MR variability The young age group had a significantly higher mean log MR than the middle and older age groups, as shown in Table I, with geometric means of 1.95, 1.45 and 1.22, respectively. Sex, CYP2C19 inhibitors and urine pH were not significantly related to the imipramine MR; all relevant two-sample t-tests produced results of P . 0.05. However, proton-pump inhibitors (PPIs), when tested alone without the other CYP2C19 inhibitors, were significantly associated with lower imipramine MR (1.12 vs. 1.50, P ¼ 0.02). The patients had the following PPIs in their medication list: omeprazole (20), esomeprazole (17), pantoprazole (4), rabeprazole (3) and lansoprazole (1). The young group also had a significantly lower sum of I þ D in the urine compared with the middle age group, with geometric means in the young, middle and older age groups of 3.24, 4.98 and 4.35, respectively (P ¼ 0.018). When assessing the sum of I þ D, females had higher values than males, 5.06 + 3.26 vs. 3.81 + 3.43 nmoles/g cr (P ¼ 0.0062). Urine pH showed a significant negative relationship with I þ D, but the correlation was weak (R 2 ¼ 0.013).
Concomitant medications About 30% of patients were positive for one or more of the following: hydrocodone, oxycodone, hydromorphone or oxymorphone. Fourteen percent were positive for morphine, and 9% were positive for methadone. The concomitant opioid frequencies are shown in Table II. Evaluation of the individual opioids for their impact on the imipramine MR produced no significant results. However, imipramine patients with no reported opioids in the medication list had a significantly lower geometric mean MR than the opioid-reporting population, as shown in Table II. Furthermore, patients with two or more reported opioids had a significantly higher MR than those with only one reported opioid (1.71 vs. 1.45, P , 0.05). A slightly, but not significantly, higher mean MR was found in subjects positive for oxymorphone (P ¼ 0.07), with geometric mean MRs of 1.59 and 1.40, respectively. A slightly higher MR was also seen for patients positive for hydromorphone (P ¼ 0.09), where the respective means were 1.57 and 1.41. Other common concomitant drugs, including gabapentin, carisoprodol and benzodiazepines, were not significantly related to the MR (P . 0.05).
Discussion The results of this study help establish observed normal ranges for urinary concentrations of imipramine and desipramine. The mean desipramine urine concentration was slightly higher than
Figure 1. (a) Log creatinine-corrected imipramine concentrations. The median (interquartile range) imipramine urine concentrations were 0.484 mg/g creatinine (0.187–1.11). (b) Log creatinine-corrected desipramine concentrations. The median (interquartile range) desipramine urine concentrations were 0.712 mg/g creatinine (0.252–1.82). (c) Log creatininecorrected MRs. The median (interquartile range) urine MRs were 1.43 (0.754– 2.77). (d) Relationship between imipramine concentration and MR. The MR showed a weak negative relationship with the imipramine urine concentration (y ¼ 20.21x ¼ 0.098, R 2 ¼ 0.068, P , 0.0001). GM, geometric mean.
the mean imipramine urine concentration. Desipramine plasma half-life has a wide reported range of 7 – 60 h, and imipramine plasma half-life is 8 – 16 h (6). The wider range of desipramine plasma half-life may be due to the variability in hepatic metabolism (6). Imipramine also undergoes hepatic metabolism; but has multiple CYP enzymes involved, leading to a wide bioavailability range after oral administration of 22 –77%. Enterohepatic circulation may occur for both drugs. Elimination rates from the plasma will include metabolism and urinary excretion, whereas elimination rates in the urine represent only the fraction of the drug and drug metabolites that get excreted via the urine. Less than 5% of imipramine is excreted unchanged in the urine (6) and desipramine fraction excreted unchanged is ,3%, although this varies between different ethnic groups (17). The scatterplot in Figure 2 shows a positive relationship between desipramine and imipramine urine concentrations. The parent drug and metabolite were moderately positively correlated, with an R 2-value of 0.52. Besides demethylation to desipramine by
CYP2C19, CYP1A2 and CYP3A4, imipramine undergoes hydroxylation by CYP2D6 to 2- and 10-hydroxyimipramine. No evidence of saturation of metabolism was noted in this study. This is not surprising for two main reasons. First, imipramine is metabolized by multiple CYP enzymes and secondly desipramine concentrations, not imipramine concentrations, have been found to increase with an increasing imipramine dose (18).
Age The young age group had a significantly higher mean log MR than the middle and older age groups, which agrees with previous studies on concentrations in plasma. Studies in children have reported higher concentrations of TCA metabolites compared with adults (19 – 21). The young also had the lowest total amount of active drug in urine, imipramine plus desipramine. Steady-state plasma concentrations of imipramine were found to be higher in an older population, ranging from 50 to 65 year Metabolism of Imipramine and Its Metabolite Desipramine 371
Figure 2. Relationship between imipramine and desipramine. Urine desipramine is positively correlated with urine imipramine concentrations, but the slope of 0.79 shows that they are not directly proportional (y ¼ 0.79x þ 0.098, R 2 ¼ 0.52, P , 0.0001).
Table I Effect of Various Factors on MR
Age Young (Y, 18– 36 years) Middle (M, 37–66 years) Old (O, 67–90 years) Gender Male Female CYP2C19 inhibition Inhibition No inhibition PPIs PPI prescribed No PPI prescribed Opioids Opioids prescribed No opioids prescribed 2þ opioids Two or more opioids prescribed One opioid prescribed
N
Geometric mean MR
Significance
70 417 97
1.95 1.45 1.22
Y–M: P ¼ 0.047; Y–O: P ¼ 0.005; M–O: P ¼ 0.241
201 397
1.38 1.51
P ¼ 0.125
71 529
1.31 1.49
P ¼ 0.145
45 555
1.12 1.50
P ¼ 0.024
476 46
1.54 1.03
P ¼ 0.004
173 303
1.71 1.45
P ¼ 0.039
Table II Type and Frequency of Concomitant Opioids Opioid
# Subjects with drug in the medication list
# Subjects with drug in urinea
% Subjects with drug in the medication list
% Subjects with drug in urinea
Oxycodone Hydrocodone Morphine Fentanyl Tramadol Methadone Oxymorphone Buprenorphine Hydromorphone Codeine Propoxyphene Meperidine Opium
208 200 73 45 34 33 23 18 12 10 4 1 1
186 190 81 41 27 56 196 15 205 10 1 2 N/A
35 33 12 8 6 6 4 3 2 2 0.7 0.2 0.2
31 32 14 7 5 9 33 3 34 2 0.2 3 0
a
Concentration of drug is above the cutoff value.
372 Ramey et al.
old, than a younger population ranging from 30 to 39 year old (22). One explanation for this finding is that younger people have a more rapid demethylation pathway than older people, whereas the hydroxylation pathways are comparable. Possible explanations for this include age-related changes in enzyme function and changes in hepatic blood flow (22). Enzyme function may be decreased for several reasons. Decreased quantities of hepatic smooth endoplasmic reticulum with age may contribute to reduced activity of NADPH-cytochrome P450 reductase, or decreased quantity of cytochrome P450 enzymes, or both (23). The decrease in metabolism appears to affect demethylation of imipramine more than hydroxylation of desipramine, leading to higher concentrations of the parent drug and a lower MR. Studies have also found a decline in imipramine clearance in the elderly with no change in bioavailability, suggesting that this is due to impaired demethylation to desipramine (23). Another study, however, has shown a disproportionate increase in desipramine levels when imipramine dose was increased (18), suggesting that hydroxylation, and not demethylation, was the saturable pathway. While both pathways are variable between patients, the overall consensus on imipramine metabolism is that it is impaired in the elderly, which is consistent with the findings in this study. No significant difference in the MR was found between men and women, which is consistent with previous studies (19, 22). This study did find a higher sum of imipramine plus desipramine in women compared with men. Some studies have found higher TCA concentrations in women, but these findings have been attributed to concomitant medications such as oral contraceptives, which can inhibit hepatic enzymes such as CYP3A (17). The lack of MR difference may also be due to CYP2D6 inhibition, which would result in expected increases in urinary concentrations of both imipramine and desipramine, thus resulting in no change in the overall MR.
Proton-pump inhibitors The PPIs, that is omeprazole, esomeprazole, lansoprazole, pantoprazole and rabeprazole, are known inhibitors of CYP2C19 (13). All PPIs are extensively metabolized in the liver, with the CYP2C19 enzyme being a major metabolic pathway that is in turn inhibited from metabolizing other drugs (24). Lansoprazole and omeprazole are the most potent inhibitors of CYP2C19, while rabeprazole and pantoprazole are the least potent inhibitors (24). Inhibition constants (Ki) for the PPIs are as follows: lansoprazole 0.4–1.5 mM, omeprazole 2–6 mM, esomeprazole 8 mM, pantoprazole 14 – 69 mM and rabeprazole 17 – 21 mM (25). A decrease in MR for patients with a PPI in the medication list can be explained by inhibition of demethylation of imipramine, leading to increased imipramine and thus lower MR. One would expect other CYP2C19 inhibitors to lower MR, but this was not the case in the present study. One possible explanation is that urinary MRs are not robust to detect subtle changes via CYP2C19 inhibition.
Opioids Patients with opioids contained in their medication list had a significantly higher MR than those without concomitant opioids (1.54 vs. 1.03, P ¼ 0.004). While tests assessing each individual
opioid were not significant in their effect on imipramine MR, opioids in general may increase it. It should be noted that inaccuracies in the physician-reported medication list were a possibility, and some lists (78) were not filled out. One possible explanation for this increased MR is the inhibition of desipramine metabolism by CYP2D6. Tramadol, codeine, oxycodone and hydrocodone are all substrates of CYP2D6 (23), and methadone is a known competitive inhibitor of CYP2D6 (26). Also, drugs metabolized by CYP2D6 often follow nonlinear pharmacokinetics due to the enzyme’s saturable nature (27, 28). In addition, genetic polymorphisms of CYP2D6 exist, with .80 known variant alleles. Several CYP2D6 variant alleles are known to result in enzyme activity that may be normal, increased, decreased or even absent (27, 29). The prevalence of these mutations differs among ethnic groups, with 7% of Caucasians and 1 –3% of other ethnic groups known to have limited CYP2D6 function (28). While this is only one possible explanation that was seen as a class effect, there is a need for further research in this matter. Physicians may want to proceed with caution when prescribing opioids concurrently with imipramine. Impairment of metabolism due to either impaired CYP2D6 function or concomitant use of medications that are CYP2D6 substrates or inhibitors may result in increased levels of desipramine. Elevated TCA plasma concentrations lead to increased adverse effects and can lead to serious events such as prolonged cardiac QT interval, central nervous system effects, coma and even death (30, 31). The decreased availability of CYP2D6 may also cause a slight increase in imipramine plasma concentration, as this is also a substrate of the enzyme. Patients taking the aforementioned opioids may need a lower imipramine dose to prevent increased plasma concentrations of both the parent drug and the metabolite.
While individual opioids alone were not significantly associated with the MR, patients positive for oxymorphone or hydromorphone had a slightly increased MR. Patients with no opioids in their medications list had a significantly lower MR than those with prescribed opioids. Furthermore, patients with only one prescribed opioid had a lower MR than those with two or more prescribed opioids. This suggests that opioids may inhibit CYP2D6 metabolism of desipramine or induce N-demethylation of imipramine, leading to higher concentrations of this active metabolite. This analysis of urinary excretion ranges of imipramine and desipramine establishes typical urinary concentrations for pain patients and discusses variables that can alter those ranges. Physicians may want to closely monitor imipramine doses in younger patients, female patients and patients taking concomitant opioids or PPIs. Adverse effects have been shown to occur over very wide ranges of both imipramine and desipramine plasma concentrations, but the ranges listed here may be useful to clinical providers as a reference.
Limitations A key limitation of this study is that dose amount and time of dose administration were not known. Changes in MR over time could be due to differences in postdose sampling times and differences in half-lives of the two compounds. Additional limitations include the lack of CYP 2D6 genotype information, which could help interpret MR findings. Finally, the lack of plasma concentration data precludes any direct comparisons between urine findings from this study and expected plasma concentrations. Although these limitations must be taken into consideration when interpreting these results, advantages of this study were a large sample size and robust quantitative drug and metabolite assay results within each subject.
References
Conclusion Urinary concentrations of imipramine and desipramine display large variability both in and among patients. The MR (desipramine/imipramine) was increased in young patients (18–36 year old) compared with both middle (37 – 66 year old) and older (67–90 year old) populations. At the same time, the sum of imipramine plus desipramine in urine was significantly lower for young patients. Sex and CYP2C19 inhibitors were not significantly related to the MR, but PPIs alone were associated with lower MR, possibly due to decreased imipramine metabolism. Females had significantly higher sums of imipramine plus desipramine in urine.
Acknowledgments K.R. was awarded a student fellowship stipend to participate in this project, supported by the UC San Diego Skaggs School of Pharmacy and Pharmaceutical Sciences and an unrestricted gift to the UC San Diego Skaggs School of Pharmacy and Pharmaceutical Sciences from Millennium Research Institute. The authors thank Dr Amadeo J. Pesce for his expert advice and guidance in this project.
Conflict of interest J.D.M. is a paid consultant for Millennium Laboratories.
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