Journal of Antimicrobial Chemotherapy (1979) 5, 601-607
A pharmacological and in vitro comparison of three oral cephalosporins
R. Wise, J. M. Andrews Department of Medical Microbiology, Dudley Road Hospital, Birmingham B18 7QH, England
S. Dean, P. G. Welling and M. J. Kendall Department of Therapeutics and Clinical Pharmacology, Queen Elizabeth Hospital, Birmingham B15 2TH, England
The pharmacology of cephradine, cephalexin and a new oral cephalosporin, cefaclor, has been compared in six volunteers. Cefaclor was absorbed rapidly and was cleared from the serum more rapidly than the other two agents. This was probably partially due to its instability in serum at body temperature, which was investigated. Against a wide range of common pathogens cefaclor was the more active oral cephalosporin. In particular the activity against Neisseria gonorrhoeae and Haemophilus influenzae was of interest Introduction Despite their good absorption and wide spectrum, the oral cephalosporins are generally less active than their parenteral counterparts (Kayser, 1971; Eykyn et al., 1976; Selwyn, 1976). In particular the two previously available agents, cephradine and cephalexin have poor activity against Haemophilus influenzae (Bill & Washington, 1977)—a common respiratory pathogen which is of particular interest to clinicians as ampicillin resistance is increasingly being recognized (Smith, 1976; Williams & Cavanagh, 1974). In this study the pharmacokinetics and antibacterial spectrum of cephradine and cephalexin were compared with a new agent, cefaclor, an orally absorbed semi-synthetic antibiotic with the formula 3-chloro-7-D-(2 phenylglycinamide)-3-cephem-4 carboxylic acid, see Figure 1. In addition the activity of those oral antimicrobials commonly used to treat chest infections was compared with these three cephalosporins. The in vitro stability of the three cephalosporins was also compared. Methods Six volunteers, three male, were given 500 mg single doses of all three drugs administered one week apart in a randomized CTOSS-OVCT design. Eleven blood samples at the times 601 03O5-7453/79/05O6O1 + 07 S01.00/0 © 1979 The British Society for Antimicrobial Chemotherapy
602
R. Wise et al.
shown in Table I, were drawn immediately before and serially after each dose and urine was collected for 12 h. Drug degradation in the serum and urine was minimized by immediate freezing to — 20°C after collection. All dosing and sampling procedures were carried out at the Queen Elizabeth Hospital, Birmingham. Samples were transported frozen to the Microbiology Department of Dudley Road Hospital, Birmingham and were assayed within 24 h. A standard large plate diffuse assay was used with standards and samples applied in triplicate. The indicator organism was a Bacillus subtilis. The 95 % confidence limits of the assay were ±16%.
CH
com
COOH
Figure 1. The structure of cefaclor.
The stability of cephalexin, cephradine and cefaclor in pooled human serum of pH 7-2 was compared by assaying a spiked sample, containing 15 mg/1 which was kept at 37°C and at — 20°C, at daily intervals up to seven days, (and in the case of cefaclor assays were performed at 4 hourly intervals on day one). The standards for the assay were freshly prepared daily (on day one for cefaclor the samples were kept at — 20°Q. The stability of cefaclor in 'Isosensitest' agar was studied by incorporating 20, 10 and 5 mg/1 into the media, pouring into 25 X 25 cm assay plates and incubating at 37°C. Standard solutions prepared at the same time were frozen at — 20°C. At 2, 4, 6 and 24 h, 1 cm diameter plugs of agar were removed with a cork-borer and placed on antibioticfree media. The frozen standards were thawed and incorporated into agar plates of similar depth. Plugs were similarly removed from the standard agar and placed onto the antibiotic-free media. The assay plates were then incubated at 37°C for 18 h and the zone sizes recorded. This enabled the rate of decline of cefaclor in 'Isosensitest' agar to be measured. A wide range of recently isolated bacteria were tested against cefaclor, cephalexin and cephradine. A routine agar plate dilution procedure was used with the antimicrobial agents incorporated into 'Isosensitest' agar (Oxoid) pH 7-2, at concentrations from 0004 to 128 mg/1. The media were supplemented as follows: chocolate agar for Neisseria gonorrhoeae; 10% lysed human blood for streptococci and Bacteroides fragilis; and Levinthal agar for H. influenzae. The organisms were grown overnight in either liquid or on solid media. Two inocula were then prepared by dilution so that 1 ul transferred to the plate represented an inoculum of 10* or 10s colony forming units (c.f.u.). The plates were incubated at 37°C for 18 h, (H. influenzae and N. gonorrhoeae in 10% CO», and B. fragilis in a 'GasPak' anaerobic system) and the minimum inhibitory concentration (MIC) was taken as the mg of antimicrobial per litre of media which caused an estimated 99 % reduction in the initial inoculum. Using identical techniques as described above the MICs of ampicillin, amoxycillin, trimethoprim, sulphamethoxazole, tetracycline, erythromycin and chloramphenicol against 55 strains of H. influenzae and 30 strains of Streptococcus pneumoniae were obtained.
% in urine ± S.D.
S.D.
% in urine ± S.D. Cefaclor Mean
S.D.
% in urine ±S.D. Cephalexin Mean
S.D.
Cephradine Mean
0-5 0-8
0-8 0-9
0-9 11
IS min
6-1 2-7
6-6 51
5-9 4-0
30 min
9-2 1-7
12-9 2-7
9-4 4-7
1h
6-9 2-5
11-3 3-4
9-4 2-8
1-5 h
4-3 1-7
8-6 3-4
9-7 3-2
2h
1-6 0-7
5-3 2-5
4-5 11
3h
0-3 0-4
2-4 1-2
1-9 0-5
4h
0 0
08 0-9
0-7 0-6
5h
Table L Mean serum levels (mg/1) of cephradine, cephalexin and cefaclor obtained from six volunteers
0 0 54±15
0-2 0-5 85±9
01 0-2 86±15
6h
54±15
86±9
88 ±15
12 h
604
Wise et al.
Results Pharmacology Mean serum levels obtained in the six subjects, together with standard deviations, are shown in Table I. Cefaclor and cephalexin were absorbed into the circulation somewhat faster than cephradine, while cefaclor was excreted more rapidly than the other two compounds. Levels obtained by cefaclor were not significantly different from cephradine and cephalexin until 1-5 h after dosing. Five hours after the dose was given cefaclor was completely cleared from serum while levels of cephradine and cephalexin were still detectable at 6 h. Peak serum levels of cefaclor (9-2 mg/1) were similar to those of cephradine (9-7 mg/1), but lower than those of cephalexin (12-9 mg/1). However, areas under serum level curves, calculated by the trapezoidal rule and also by conventional curve stripping methods, showed no differences between compounds when normalized for the rate of drug clearance, indicating similar absorption efficiencies for the three compounds. Urinary recovery of cefaclor, being only 54 % of the administered dose, was significantly lower than that of cephradine and cephalexin. The in vitro serum half-lives of the three cephalosporins at 37°C were as follows: cefaclor 2-4 h, cephradine 54 h and cephalexin 60 h. At — 20°C the half-lives were greater than 7 days in each case. Antibacterial spectrum The in vitro activities at two inoculum levels are shown in Table II. Against Gramnegative bacteria cefaclor was the most active agent, with cephradine usually the least active. For example, against non-P-lactamase producing strains of Escherichia coli Table IL Geometric mean MIC mg/1 strains tested E.coliV-* E. coli P + * Klebsiella spp P. mirabilis P P. mirabilis P + Indole positive Proteus B. fiagilis N. gonorrhoeae P —
Grp. D. Streptococci Grp. A. Streptococci S. aureus P— S. aureus P+ Enterobacter spp H. influenzae P — H. influenzae p + Strept. pneumoniae
22 28 50 37 15 20 10 31 15 19 23 25 12 44 11 30
L + = l o w inoculum (10* c.f.u.). H + = high inoculum (10 § c.f.u.). P— * = non-P-lactamase producer. P + * = p-lactamase producer.
cephalexin
cephradine
H+
L
H
L
7-5 16 6-4 8-4 76 128 128 0-051 128 0-62 4-8 171 107 3-4 18-6 0-5
6 6-7 4-6 10 32 84 28 0-26 95 0-41 1-7 1-8 24 8 8-5 0-9
14 16 6-4 22 90 128 84 0-58 105 0-43 4 6 128 42 32 11
8-5 8 8 14 48 51 21 0-29 52 0-3 1-6 1-5 24 13-9 11-3 0-46
cefaclor L+ 2-4 2-8 1-9 1-8 14 28 128 0O35 128 0-39 1-8 1-6 13 3-0 2-4 0-4
H 23-1 23-4 11-3 45 128 128 59 0-45 52 0-37 4 6 128 14-9 19-6 11
Comparison of three oral cephalosporins
605
cefaclor was at least twice as active as cephalexin and three times as active as cephradine. Against the plasmid mediated P-lactamase producing strains of E. coli, the Klebsiella spp. and Proteus mirabilis differences were less apparent at the higher inoculum but with cefaclor somewhat more active. Indole positive Proteus spp. and the Enterobacter spp. were more resistant to all three cephalosporins with cefaclor the most active, but a considerable inoculum effect was seen with all three drugs, and there was no evidence of useful activity against Group D streptococci or B. fragilis. There were considerable differences in activity against N. gonorrhoeae with cefaclor being about ten times more active than the other two agents. One potent P-lactamase producing strain (penicillin MIC 128 mg/1) tested at the higher inoculum gave MICs for cephradine, cephalexin and cefaclor of 4, 1 and 0-5 mg/1 respectively. On the other hand, at the high inoculum against the strains of p-lactamase producing Staphylococcus aweus cefaclor was the least active in vitro. Non-P-lactamase producing staphylococci and Group A streptococci were equally sensitive to all three antibiotics. Against non-P-lactamase producing H. influenzae cefaclor was two to four times more active than cephalexin or cephradine and it was still the most active against p-lactamase producing strains despite showing a sevenfold inoculum effect compared with fourfold for cephalexin and twofold for cephradine, and against Strept. pneumoniae, cefaclor was the most active agent. Table IH. Activity of ten antimicrobial agents against Strept. pneumoniae and H. influenzae
Antibiotic
cefaclor amoxycillin ampicillin tetracycline erythromycin
tnmethoprim sulphamethoxazole chloramphenicol cephalexin cephradine
Inoculum effect Proportion of strains Level mg/1* on P-lactamase Usual approximating producing (60) (30) dose to 50% peak strains serum levels Strept. pneumoniae H. influenzae H. influenzae 500 mg 250 mg 250 mg 250 mg 500 mg 160 mg 800 gm 500 mg 500 mg 500 mg
4 4 2 2 1 2 32 4
6 4
100% 100% 95% 70% 100% 86% 77% 90% 100% 100%
93% 95% 85% 92% 8% 100% 100% 100% 0 0
X4-8
x64 x64 x2 xl-2
x4 xl6
x2 x4 x2
*Levels obtained from Nightingale, Greene & Quintiliani (1975), Fowle, (1973), Wise (1978).
The comparison of cefaclor with other antimicrobials against respiratory pathogens is shown in Table HI, where the percentage of strains inhibited by 50 % of the reported peak concentrations of the different agents is shown. Cefaclor at 4 mg/1 inhibited over 90% of all strains of Strept. pneumoniae and H. influenzae tested. Amoxycillin at 4 mg/1 and ampicillin at 2 mg/1 also gave good results against both organisms but were very susceptible to the inoculum effect with P-lactamase producing H. influenzae. Tetracycline, trimethoprim and sulphamethoxazole showed poor activity against Strept. pneumoniae but had high activity against H. influenzae, while in the case of erythromycin the situation was reversed. Chloramphenicol was consistently the most active compound showing very little inoculum effect.
606
R. Wise et al.
In Figure 2 the rate of loss of activity of cefaclor in 'Isosensitest' agar is shown. Over 0 to 6 h there is an almost linear loss of activity with time. The half-life of cefaclor in this agar was 5-2 h.
Figure 2. The decrease of antimicrobial activity of cefaclor, in Isosensitest agar at 37°C, with time.
Discussion The pharmacology of the three oral cephalosporins is similar. Cephalexin gave slightly higher peak levels, whilst cefaclor produced the least variation in different subjects. Cefaclor was both absorbed and cleared faster than the other two agents. The more rapid disappearance from the serum of cefaclor must be partially due to the instability of this compound in serum. In this study particular attention was paid to the freezing of all samples immediately after the serum was separated. Glynne et al. (1978) have reported somewhat higher urinary recoveries of cefaclor after a 250 mg dose even though the sampling periods were similar. It is possible that variations in urinary pH may well affect the apparent recovery of cefaclor from urine (data from Eli Lilly). In antibacterial spectrum, none of the agents was very active against indole positive Proteus spp., Enterobacter spp., Strept. faecalis or B. fragilis. Otherwise cefaclor performed well against a wide range of pathogens, and except against a high inoculum of P-lactamase producing Staph. aweus was consistently the most active of the three agents tested. We weTe particularly interested to note that although Strept. pneumoniae were susceptible to all three agents, only cefaclor shows useful activity gainst H. influenzae, the fact which caused the study to be extended to compare it with other oral antibiotics currently in use to treat chest infections. It is also interesting to speculate that the in vitro activity of cefaclor may be underestimated by the MIC determinations as performed in this study. Cefaclor is unstable in agar as well as in serum and declining with time the true activity at time 0, rather than the concentration at the time killing occurs, will be higher. It is not known what period of time is critical for the killing process. It is also not known what relationship the cefaclor MIC has to this process. In the case of ampicillin it is probable that at a concentration of twice the MIC, killing occurs at about 1 to 2 h after exposure (Rolinson, MacDonald & Wilson, 1977). In the case of cephalexin a longer period of time is necessary. It is doubtful if the instability in agar would lead to a greater than two-fold decrease in apparent antimicrobial activity. We have attempted to compare the antimicrobials by noting the percentage of strains inhibited by an amount of compound approximating to 50% of the average peak serum level. Such an exercise will only give an imprecise comparison and minor differences as seen, for example, between ampicillin and amoxycilh'n have little relevance.
Comparison of three oral cephalosporins
607
The pharmacokinetic differences between the three oral cephalosporins are relatively minor and the lower peak serum values obtained in the case of cefaclor may well be compensated by the superior in vitro activity against a wide range of common pathogens. References Bill, N. J. & Washington, J. A. Comparison of in vitro activity of cephalexin, ccphradine and cefaclor. Antimicrobial Agents and Chemotherapy 11: 470-74 (1977). Eykyn, S., Jenkins, C , King, A. & Phillips, I. Antibacterial activity of cefuroxime, a new cephalosporin antibiotic, compared with that of cephaloridine, cephalothin and cefamandole. Antimicrobial Agents and Chemotherapy 9: 689-95 (1976). Fowle, A. S. E. Aspects of the pharmacokinetic behaviour of trimethoprim and sulphamethoxazole. In Trimethoprimlsulphamethoxazole in Bacterial Infections. (Bernstein, L. S. & Salter, A. J., eds). Churchill Livingstone, London (1973), pp. 63-72. Glynne, A., Goulbourn, R. A. & Ryden, R. A human pharmacology study of cefaclor. Journal of Antimicrobial Chemotherapy 4: 343-8 (1978). Kayser, F. H. In vitro activity of cephalosporin antibiotics against Gram-positive bacteria. Postgraduate Medical Journal (Feb. Suppl.) 47: 14-20 (1971). Nightingale, C. H., Greene, D. S. & QuinuJiani, R. Pharmacokinetics and clinical use of cephalosporin antibiotics. Journal of Pharmaceutical Science 64: 1899-926 (1975). Rolinson, G. N., MacDonald, A. C. & Wilson, D. A. Bactericidal action of 3-lactam antibiotics on E. coli with particulr reference to ampicfllin and amoxycillin. Journal of Antimicrobial Chemotherapy 3: 541-53 (1977). Selwyn, S. Rational choice of penicillins and cephalosporins based on parallel in vitro and in vivo tests. Lancet ii: 616 (1976). Smith, A. L. Antibiotics and invasive H. influenzae. New England Journal of Medicine 294: 1329-31, (1976). Wise, R. Table of expected concentrations of antibiotics. In Antibacterial Chemotherapy (Reeves, D. S., Phillips, I., Williams, J. D. & Wise, R., eds). Churchill Livingstone, London (1978), pp. 151-6. Williams, J. D., & Cavanagh, P. Ampicillin-resistant H. influenzae meningitis. Lancet i: 864 (1974). (Manuscript accepted 26 January 1979)
A pharmacological and in vitro comparison of three oral cephalosporins
R. Wise, J. M. Andrews Department of Medical Microbiology, Dudley Road Hospital, Birmingham B18 7QH, England
S. Dean, P. G. Welling and M. J. Kendall Department of Therapeutics and Clinical Pharmacology, Queen Elizabeth Hospital, Birmingham B15 2TH, England
The pharmacology of cephradine, cephalexin and a new oral cephalosporin, cefaclor, has been compared in six volunteers. Cefaclor was absorbed rapidly and was cleared from the serum more rapidly than the other two agents. This was probably partially due to its instability in serum at body temperature, which was investigated. Against a wide range of common pathogens cefaclor was the more active oral cephalosporin. In particular the activity against Neisseria gonorrhoeae and Haemophilus influenzae was of interest Introduction Despite their good absorption and wide spectrum, the oral cephalosporins are generally less active than their parenteral counterparts (Kayser, 1971; Eykyn et al., 1976; Selwyn, 1976). In particular the two previously available agents, cephradine and cephalexin have poor activity against Haemophilus influenzae (Bill & Washington, 1977)—a common respiratory pathogen which is of particular interest to clinicians as ampicillin resistance is increasingly being recognized (Smith, 1976; Williams & Cavanagh, 1974). In this study the pharmacokinetics and antibacterial spectrum of cephradine and cephalexin were compared with a new agent, cefaclor, an orally absorbed semi-synthetic antibiotic with the formula 3-chloro-7-D-(2 phenylglycinamide)-3-cephem-4 carboxylic acid, see Figure 1. In addition the activity of those oral antimicrobials commonly used to treat chest infections was compared with these three cephalosporins. The in vitro stability of the three cephalosporins was also compared. Methods Six volunteers, three male, were given 500 mg single doses of all three drugs administered one week apart in a randomized CTOSS-OVCT design. Eleven blood samples at the times 601 03O5-7453/79/05O6O1 + 07 S01.00/0 © 1979 The British Society for Antimicrobial Chemotherapy
602
R. Wise et al.
shown in Table I, were drawn immediately before and serially after each dose and urine was collected for 12 h. Drug degradation in the serum and urine was minimized by immediate freezing to — 20°C after collection. All dosing and sampling procedures were carried out at the Queen Elizabeth Hospital, Birmingham. Samples were transported frozen to the Microbiology Department of Dudley Road Hospital, Birmingham and were assayed within 24 h. A standard large plate diffuse assay was used with standards and samples applied in triplicate. The indicator organism was a Bacillus subtilis. The 95 % confidence limits of the assay were ±16%.
CH
com
COOH
Figure 1. The structure of cefaclor.
The stability of cephalexin, cephradine and cefaclor in pooled human serum of pH 7-2 was compared by assaying a spiked sample, containing 15 mg/1 which was kept at 37°C and at — 20°C, at daily intervals up to seven days, (and in the case of cefaclor assays were performed at 4 hourly intervals on day one). The standards for the assay were freshly prepared daily (on day one for cefaclor the samples were kept at — 20°Q. The stability of cefaclor in 'Isosensitest' agar was studied by incorporating 20, 10 and 5 mg/1 into the media, pouring into 25 X 25 cm assay plates and incubating at 37°C. Standard solutions prepared at the same time were frozen at — 20°C. At 2, 4, 6 and 24 h, 1 cm diameter plugs of agar were removed with a cork-borer and placed on antibioticfree media. The frozen standards were thawed and incorporated into agar plates of similar depth. Plugs were similarly removed from the standard agar and placed onto the antibiotic-free media. The assay plates were then incubated at 37°C for 18 h and the zone sizes recorded. This enabled the rate of decline of cefaclor in 'Isosensitest' agar to be measured. A wide range of recently isolated bacteria were tested against cefaclor, cephalexin and cephradine. A routine agar plate dilution procedure was used with the antimicrobial agents incorporated into 'Isosensitest' agar (Oxoid) pH 7-2, at concentrations from 0004 to 128 mg/1. The media were supplemented as follows: chocolate agar for Neisseria gonorrhoeae; 10% lysed human blood for streptococci and Bacteroides fragilis; and Levinthal agar for H. influenzae. The organisms were grown overnight in either liquid or on solid media. Two inocula were then prepared by dilution so that 1 ul transferred to the plate represented an inoculum of 10* or 10s colony forming units (c.f.u.). The plates were incubated at 37°C for 18 h, (H. influenzae and N. gonorrhoeae in 10% CO», and B. fragilis in a 'GasPak' anaerobic system) and the minimum inhibitory concentration (MIC) was taken as the mg of antimicrobial per litre of media which caused an estimated 99 % reduction in the initial inoculum. Using identical techniques as described above the MICs of ampicillin, amoxycillin, trimethoprim, sulphamethoxazole, tetracycline, erythromycin and chloramphenicol against 55 strains of H. influenzae and 30 strains of Streptococcus pneumoniae were obtained.
% in urine ± S.D.
S.D.
% in urine ± S.D. Cefaclor Mean
S.D.
% in urine ±S.D. Cephalexin Mean
S.D.
Cephradine Mean
0-5 0-8
0-8 0-9
0-9 11
IS min
6-1 2-7
6-6 51
5-9 4-0
30 min
9-2 1-7
12-9 2-7
9-4 4-7
1h
6-9 2-5
11-3 3-4
9-4 2-8
1-5 h
4-3 1-7
8-6 3-4
9-7 3-2
2h
1-6 0-7
5-3 2-5
4-5 11
3h
0-3 0-4
2-4 1-2
1-9 0-5
4h
0 0
08 0-9
0-7 0-6
5h
Table L Mean serum levels (mg/1) of cephradine, cephalexin and cefaclor obtained from six volunteers
0 0 54±15
0-2 0-5 85±9
01 0-2 86±15
6h
54±15
86±9
88 ±15
12 h
604
Wise et al.
Results Pharmacology Mean serum levels obtained in the six subjects, together with standard deviations, are shown in Table I. Cefaclor and cephalexin were absorbed into the circulation somewhat faster than cephradine, while cefaclor was excreted more rapidly than the other two compounds. Levels obtained by cefaclor were not significantly different from cephradine and cephalexin until 1-5 h after dosing. Five hours after the dose was given cefaclor was completely cleared from serum while levels of cephradine and cephalexin were still detectable at 6 h. Peak serum levels of cefaclor (9-2 mg/1) were similar to those of cephradine (9-7 mg/1), but lower than those of cephalexin (12-9 mg/1). However, areas under serum level curves, calculated by the trapezoidal rule and also by conventional curve stripping methods, showed no differences between compounds when normalized for the rate of drug clearance, indicating similar absorption efficiencies for the three compounds. Urinary recovery of cefaclor, being only 54 % of the administered dose, was significantly lower than that of cephradine and cephalexin. The in vitro serum half-lives of the three cephalosporins at 37°C were as follows: cefaclor 2-4 h, cephradine 54 h and cephalexin 60 h. At — 20°C the half-lives were greater than 7 days in each case. Antibacterial spectrum The in vitro activities at two inoculum levels are shown in Table II. Against Gramnegative bacteria cefaclor was the most active agent, with cephradine usually the least active. For example, against non-P-lactamase producing strains of Escherichia coli Table IL Geometric mean MIC mg/1 strains tested E.coliV-* E. coli P + * Klebsiella spp P. mirabilis P P. mirabilis P + Indole positive Proteus B. fiagilis N. gonorrhoeae P —
Grp. D. Streptococci Grp. A. Streptococci S. aureus P— S. aureus P+ Enterobacter spp H. influenzae P — H. influenzae p + Strept. pneumoniae
22 28 50 37 15 20 10 31 15 19 23 25 12 44 11 30
L + = l o w inoculum (10* c.f.u.). H + = high inoculum (10 § c.f.u.). P— * = non-P-lactamase producer. P + * = p-lactamase producer.
cephalexin
cephradine
H+
L
H
L
7-5 16 6-4 8-4 76 128 128 0-051 128 0-62 4-8 171 107 3-4 18-6 0-5
6 6-7 4-6 10 32 84 28 0-26 95 0-41 1-7 1-8 24 8 8-5 0-9
14 16 6-4 22 90 128 84 0-58 105 0-43 4 6 128 42 32 11
8-5 8 8 14 48 51 21 0-29 52 0-3 1-6 1-5 24 13-9 11-3 0-46
cefaclor L+ 2-4 2-8 1-9 1-8 14 28 128 0O35 128 0-39 1-8 1-6 13 3-0 2-4 0-4
H 23-1 23-4 11-3 45 128 128 59 0-45 52 0-37 4 6 128 14-9 19-6 11
Comparison of three oral cephalosporins
605
cefaclor was at least twice as active as cephalexin and three times as active as cephradine. Against the plasmid mediated P-lactamase producing strains of E. coli, the Klebsiella spp. and Proteus mirabilis differences were less apparent at the higher inoculum but with cefaclor somewhat more active. Indole positive Proteus spp. and the Enterobacter spp. were more resistant to all three cephalosporins with cefaclor the most active, but a considerable inoculum effect was seen with all three drugs, and there was no evidence of useful activity against Group D streptococci or B. fragilis. There were considerable differences in activity against N. gonorrhoeae with cefaclor being about ten times more active than the other two agents. One potent P-lactamase producing strain (penicillin MIC 128 mg/1) tested at the higher inoculum gave MICs for cephradine, cephalexin and cefaclor of 4, 1 and 0-5 mg/1 respectively. On the other hand, at the high inoculum against the strains of p-lactamase producing Staphylococcus aweus cefaclor was the least active in vitro. Non-P-lactamase producing staphylococci and Group A streptococci were equally sensitive to all three antibiotics. Against non-P-lactamase producing H. influenzae cefaclor was two to four times more active than cephalexin or cephradine and it was still the most active against p-lactamase producing strains despite showing a sevenfold inoculum effect compared with fourfold for cephalexin and twofold for cephradine, and against Strept. pneumoniae, cefaclor was the most active agent. Table IH. Activity of ten antimicrobial agents against Strept. pneumoniae and H. influenzae
Antibiotic
cefaclor amoxycillin ampicillin tetracycline erythromycin
tnmethoprim sulphamethoxazole chloramphenicol cephalexin cephradine
Inoculum effect Proportion of strains Level mg/1* on P-lactamase Usual approximating producing (60) (30) dose to 50% peak strains serum levels Strept. pneumoniae H. influenzae H. influenzae 500 mg 250 mg 250 mg 250 mg 500 mg 160 mg 800 gm 500 mg 500 mg 500 mg
4 4 2 2 1 2 32 4
6 4
100% 100% 95% 70% 100% 86% 77% 90% 100% 100%
93% 95% 85% 92% 8% 100% 100% 100% 0 0
X4-8
x64 x64 x2 xl-2
x4 xl6
x2 x4 x2
*Levels obtained from Nightingale, Greene & Quintiliani (1975), Fowle, (1973), Wise (1978).
The comparison of cefaclor with other antimicrobials against respiratory pathogens is shown in Table HI, where the percentage of strains inhibited by 50 % of the reported peak concentrations of the different agents is shown. Cefaclor at 4 mg/1 inhibited over 90% of all strains of Strept. pneumoniae and H. influenzae tested. Amoxycillin at 4 mg/1 and ampicillin at 2 mg/1 also gave good results against both organisms but were very susceptible to the inoculum effect with P-lactamase producing H. influenzae. Tetracycline, trimethoprim and sulphamethoxazole showed poor activity against Strept. pneumoniae but had high activity against H. influenzae, while in the case of erythromycin the situation was reversed. Chloramphenicol was consistently the most active compound showing very little inoculum effect.
606
R. Wise et al.
In Figure 2 the rate of loss of activity of cefaclor in 'Isosensitest' agar is shown. Over 0 to 6 h there is an almost linear loss of activity with time. The half-life of cefaclor in this agar was 5-2 h.
Figure 2. The decrease of antimicrobial activity of cefaclor, in Isosensitest agar at 37°C, with time.
Discussion The pharmacology of the three oral cephalosporins is similar. Cephalexin gave slightly higher peak levels, whilst cefaclor produced the least variation in different subjects. Cefaclor was both absorbed and cleared faster than the other two agents. The more rapid disappearance from the serum of cefaclor must be partially due to the instability of this compound in serum. In this study particular attention was paid to the freezing of all samples immediately after the serum was separated. Glynne et al. (1978) have reported somewhat higher urinary recoveries of cefaclor after a 250 mg dose even though the sampling periods were similar. It is possible that variations in urinary pH may well affect the apparent recovery of cefaclor from urine (data from Eli Lilly). In antibacterial spectrum, none of the agents was very active against indole positive Proteus spp., Enterobacter spp., Strept. faecalis or B. fragilis. Otherwise cefaclor performed well against a wide range of pathogens, and except against a high inoculum of P-lactamase producing Staph. aweus was consistently the most active of the three agents tested. We weTe particularly interested to note that although Strept. pneumoniae were susceptible to all three agents, only cefaclor shows useful activity gainst H. influenzae, the fact which caused the study to be extended to compare it with other oral antibiotics currently in use to treat chest infections. It is also interesting to speculate that the in vitro activity of cefaclor may be underestimated by the MIC determinations as performed in this study. Cefaclor is unstable in agar as well as in serum and declining with time the true activity at time 0, rather than the concentration at the time killing occurs, will be higher. It is not known what period of time is critical for the killing process. It is also not known what relationship the cefaclor MIC has to this process. In the case of ampicillin it is probable that at a concentration of twice the MIC, killing occurs at about 1 to 2 h after exposure (Rolinson, MacDonald & Wilson, 1977). In the case of cephalexin a longer period of time is necessary. It is doubtful if the instability in agar would lead to a greater than two-fold decrease in apparent antimicrobial activity. We have attempted to compare the antimicrobials by noting the percentage of strains inhibited by an amount of compound approximating to 50% of the average peak serum level. Such an exercise will only give an imprecise comparison and minor differences as seen, for example, between ampicillin and amoxycilh'n have little relevance.
Comparison of three oral cephalosporins
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The pharmacokinetic differences between the three oral cephalosporins are relatively minor and the lower peak serum values obtained in the case of cefaclor may well be compensated by the superior in vitro activity against a wide range of common pathogens. References Bill, N. J. & Washington, J. A. Comparison of in vitro activity of cephalexin, ccphradine and cefaclor. Antimicrobial Agents and Chemotherapy 11: 470-74 (1977). Eykyn, S., Jenkins, C , King, A. & Phillips, I. Antibacterial activity of cefuroxime, a new cephalosporin antibiotic, compared with that of cephaloridine, cephalothin and cefamandole. Antimicrobial Agents and Chemotherapy 9: 689-95 (1976). Fowle, A. S. E. Aspects of the pharmacokinetic behaviour of trimethoprim and sulphamethoxazole. In Trimethoprimlsulphamethoxazole in Bacterial Infections. (Bernstein, L. S. & Salter, A. J., eds). Churchill Livingstone, London (1973), pp. 63-72. Glynne, A., Goulbourn, R. A. & Ryden, R. A human pharmacology study of cefaclor. Journal of Antimicrobial Chemotherapy 4: 343-8 (1978). Kayser, F. H. In vitro activity of cephalosporin antibiotics against Gram-positive bacteria. Postgraduate Medical Journal (Feb. Suppl.) 47: 14-20 (1971). Nightingale, C. H., Greene, D. S. & QuinuJiani, R. Pharmacokinetics and clinical use of cephalosporin antibiotics. Journal of Pharmaceutical Science 64: 1899-926 (1975). Rolinson, G. N., MacDonald, A. C. & Wilson, D. A. Bactericidal action of 3-lactam antibiotics on E. coli with particulr reference to ampicfllin and amoxycillin. Journal of Antimicrobial Chemotherapy 3: 541-53 (1977). Selwyn, S. Rational choice of penicillins and cephalosporins based on parallel in vitro and in vivo tests. Lancet ii: 616 (1976). Smith, A. L. Antibiotics and invasive H. influenzae. New England Journal of Medicine 294: 1329-31, (1976). Wise, R. Table of expected concentrations of antibiotics. In Antibacterial Chemotherapy (Reeves, D. S., Phillips, I., Williams, J. D. & Wise, R., eds). Churchill Livingstone, London (1978), pp. 151-6. Williams, J. D., & Cavanagh, P. Ampicillin-resistant H. influenzae meningitis. Lancet i: 864 (1974). (Manuscript accepted 26 January 1979)
