Clin Biochern, Vol. 24, pp. 9-14, 1991

0009-9120/91 $3.00 + .00 Copyright © 1991 The Canadian Society of Clinical Chemists.

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Pharmacology and Pharmacokinetics of Cyclosporine D. J. FREEMAN Clinical Pharmacology Resource Group, Robarts Research Institute, 100 Perth Drive, London, Ontario, Canada N6A 5K8 Cyclosporine (CsA) is a lipophilic, immunosuppressive peptide which selectively inhibits T-lymphocyte activation in response to antigen stimulation. Although it is the drug of choice in organ transplantation, its clinical use is hampered by toxicity and unpredictable pharmacokinetics. CsA absorption from the small intestine is normally incomplete and is further reduced by intestinal dysfunction and low bile flow. That which enters the systemic blood is metabolized almost entirely by the liver and excreted in the bile. Blood CsA levels are altered to a clinically relevant degree by abnormal liver function and by drugs that induce or inhibit hepatic metabolism. Intestinal absorption may also play a role in some drug interactions. The narrow therapeutic range of CsA and the various factors that alter its kinetics underlie the continuing need to monitor this drug in blood.

KEY WORDS: cyclosporine; pharmacology; pharmacokinetics; metabolism; drug interactions.

Actions of cyclosporine C

yclosporine (CsA) is a lipophilic, cyclic undecapeptide (MW 1202) produced by the fungus Tolypocladium inflatum gams. The structure is shown in Figure 1. Although CsA was initially examined for its antifungal activity, it became apparent that the peptide was a potent immunosuppressant with selective activity toward T-lymphocytes (1). The mechanisms by which CsA inhibits T-cell activation and proliferation are complex, but an important initial step appears to be the inhibition of synthesis of several lymphokines, particularly interleukin-2, which are formed in response to antigen stimulation. Interleukin-2 plays a key role in triggering a series of biochemical events which lead to upregulation of its own receptors on T-cell membranes and ultimately to cellular proliferation (2). In addition to its antifungal and immunosuppressant properties, CsA has also been shown to 1) reverse drug resistance in malignant cancer cell models by a mechanism that may involve cell membrane Pglycoproteins (3-5), 2)have antiparasitic activity against several organisms including those responsi-

Correspondence: Dr. D. J. Freeman, Clinical Pharmacology Resource Group, Robarts Research Institute, 100 Perth Drive, London, Ontario, Canada N6A 5K8. Manuscript received May 26, 1990; revised July 6, 1990; accepted August 10, 1990. CLINICAL BIOCHEMISTRY, VOLUME 24, FEBRUARY 1991

ble for malaria (6), and 3) costimulate IgE antibody production and upregulation of IgE membrane Fc receptors on T-lymphocytes without affecting other classes of humoral antibody responses (7). The immunosuppressive activity of CsA appears to be mediated by intracellular rather than cell membrane receptors. It is capable of binding to several cytosolic proteins including calmodulin (8), cyclophilin (9), and peptidyl-prolyl cis-trans isomerase (10,11). The latter is an enzyme involved in refolding of intracellular proteins (12,13), and appears to be the same protein as cyclophilin (10,11). Whether these are true receptors is uncertain, but several pieces of evidence favour the role of cyclophilin as a CsA receptor. First, the degree of immunosuppression exhibited by analogues of CsA in vitro correlate reasonably well with their ability to bind to cyclophilin (9,14). Second, the cytotoxic activity of CsA toward Neurospora crassa and Saccharomyces cerevissiae is only evident in strains that contain cyclophilin. CsA-resistant strains either do not express the protein or express a protein which is incapable of binding CsA (15). Third, cyclophilin is present in many tissues but is particularly high in lymphoid tissue and rapidly dividing tumor cells (16). The therapeutic use of CsA as an immunosuppressant in solid organ transplantation and autoimmune diseases is limited by its narrow therapeutic range and potentially serious adverse effects. Of these, nephrotoxicity is the most troublesome. Nephrotoxicity is a dose-dependentphenomenon characterized by acute and chronic phases (17). Acute toxicity is reversible and is associated with reduction in glomerular filtration, increased proximal tubular reabsorption, oliguria and hyperkalemia (17,18). An underlying cause is renal vasoconstriction and ischemia possibly resulting from an imbalance between constrictive and dilatory prostanoids (19,20). Progression from acute to chronic toxicity leads to irreversible interstitial fibrosis and nephron loss (21). Other adverse effects which have potentially serious consequences for the patient on long-term therapy with CsA are hypertension (22,23) and lipid abnormalities (24). P h a r m a c o k i n e t i c s of cyclosporine Monitoring of CsA in blood and serum is a means of reducing the risk of nephrotoxicity or rejection 9

FREEMAN

CH3~ / H C II H/C\cH

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I CO

AA

CH3 H,. I ~CH- CH2 ~'C (S) CH3

io

2

(s)

II

(s)

I

AA11

0

AA1

H

/CH3 CH2 ~,,,,H C (s) II AAZ 0

CH3 CH2 AA3

I

C0

I

AA9

N- CH 3

I

CH)-~(RIAA8 oc

CH3

AA7

H (s) I I~I=CO=C,,,H N - C H. CH3 0 : ..................... !

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0 (s) i, C"-T"-NI'H I C CH2 CH3 I

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CH3

A/kS • AA4 I (S) N~CO~C" c,,,,H J CHz I CH3 CH3 /CH CH3 ~CH3

Figure 1--Structure of cyclosporine, MW 1202. associated with inappropriate drug concentrations. However, the pharmacokinetics of CsA in man can be quite unpredictable and the interpretation of blood CsA concentrations must be made with care. ABSORPTION

Concentrations of CsA in blood are significantly influenced by factors that alter its absorption. In healthy individuals, the oral bioavailability of CsA is low (about 30%) and quite variable due mainly to poor absorption from the small intestine. In man and rat, absorption is best described by a zero-order process (dose-independent) rather than the more common first-order (dose-dependent) process (25,26). These observations have led to the idea that an absorption "window" exists in the upper part of the small intestine (25), and that a carrier-mediated transfer of CsA across the intestinal wall may occur (25). Several factors influence the absorption process including transplant type, intestinal dysfunction, and drugs. Liver transplantation is initially accompanied by poor oral CsA absorption but as liver function improves CsA bioavailability increases (27). That bile salts are responsible for the improved bioavailability was shown in studies that involved the manipulation of bile salts in the small intestine of liver transplant recipients (27) and, similarly, in dogs (28). Other conditions known to reduce CsA absorption from the intestine are chemoradiation enteritis, acute graft vs. host disease involving the

10

intestine, and diarrhea (29). Results of studies which examined the effect of food on absorption have been inconsistent. In some cases, food was shown to increase CsA bioavailability (30,31) whereas in others, food had no significant effect (32). Based on experiments on pigs in which portal and hepatic blood CsA concentrations were monitored after oral dosing, the majority of absorbed CsA diffuses directly into the portal blood (Freeman, unpublished). In rats with cannulated thoracic ducts, only 1-2% of the absorbed CsA was transported to the blood in thoracic lymph (33,34). DISTRIBUTION

The apparent volume of distribution (Vdss) of CsA is large (about 4L/kg). In the blood, CsA binds mainly to lipoproteins and erythrocytes (35,36). The temperature-dependent redistribution of CsA between erythrocytes and plasma can cause concentrations in plasma to vary if the blood from which it is prepared is not allowed time (1.5-2 h) to cool and equilibrate at room temperature. This is one of several analytical reasons for recommending the use of whole blood for monitoring CsA (37). CsA is able to partition into many body tissues because of its lipophilic nature and the presence of the ubiquitous intracellular binding protein, cyclophilin (38,39). However, brain tissue, although having a high lipid and cyclophilin content, does not normally accumulate CsA presumably because of

CLINICAL BIOCHEMISTRY, VOLUME 24, FEBRUARY 1991

PHARMACOLOGY AND PHARMACOKINETICS OF CYCLOSPORINE TABLE 1

Pharmacokinetics of Cyclosporine in Various Patient Types t~, (h)

CL (IAdKg)

Vdss (L/Kg)

F (%)

5

6.2 (4.7-12.5)

0.23 (0.17-0.33)

1.3 (0.3)

--

4

15.8 (8.4)

0.35 (0.08)

3.5 (2.7)

--

41

10.7 (4.3-53)

0.34 (0.04-1.43)

4.5 (3.6)

27.6 (21.1)

Liver failure (A)

8

20.4 (10.8--48)

0.17 (0.11-0.29)

3.9 (1.8)

--

Liver Tx (A)

6

--

0.33 (0.29-0.46)

--

(8-60)

Heart Tx (A)

4

6.4 (5.2-9.3)

0.24 (0.13-0.32)

1.3 (0.2)

38 (13.1)

Renal Tx (Ped)

7

7.3 (8.1-16.6)

0.71 (0.59-0.93)

4.7 (1.5)

31 (10.2)

Liver Tx (Ped)

10

--

0.56 (0.11-1.29)

--

(