Studies of steroid metabolism using isotopically-labeled compounds at physiological levels present unique problems in the identification of the metabolites and in the demonstration of their radiochemical purity. The submicrogram quantities of material available preclude the use of classical identification techniques. The character of the evidence obtained, the advantages and disadvantages of chromatographic and countercurrent distribution methods are discussed. Crystallization to constant specific activity is a recognized method for demonstrating that a substance is not radiochemically impure. Its parameters have never been accurately defined. Its true power is achieved only when it is preceded by extensive purification of the material to be characterized. In this way, the unknown material is first categorized by its migration rate in various solvent systems, and then by its crystalline identity with the carrier compound. The likelihood of two dissimilar steroids being both isopolar and isomorphic is held to be remote. Liquid scintillation spectrometry and gravimetry are the techniques used for the determination of constant specific activity. This method for measurement of radioactivity is extremely flexible, sensitive, and lends itself to dual-isotope experiments. Gravimetry under standardized conditions is suitably precise and much more generally applicable than spectroscopic quantitation. The parameters of the technique of rapid, forced microcrystallization are analyzed. In particular, the problem of contamination of crystals is analyzed in detail, and it is pointed out that classical concepts of purification by crystallization, developed chiefly in connection with ionic inorganic materials, must be modified when applied to nonionic steroid compounds. A mathematical analysis of the errors inherent in this technique indicates that 3 successive crystallizations of a pure radioactive compound should yield values for the specific activity which are within ± 5 % of the average of the 3 values.

The pharmacokinetics of cefotaxime after intramuscular injection and intravenous infusion were determined. The mean peak serum level after the 500-mg intramuscular dose was 11.7 ,ug/ml, and it was 20.5 ,ug/ml after a 1,000-mg dose.The serum half-life was 1.2 and 1.3 h, respectively for the two doses. The apparent fractional volumes of distribution of 32 and 37 liters were All subjects were judged healthy on the basis of history, physical examinations, chemistry profile (SMA 12/60; Technicon), complete blood count, urinalysis, and creatinine clearance. Subjects with known sensitivity to penicillins or cephalosporins were excluded. Intramuscular injection study. Fourteen subjects were divided into two groups. Group A (seven subjects) received an intramuscular injection of 500 mg of cefotaxime, followed 1 week later by a 1,000-mg injection. Group B (seven subjects) received 1,000 mg followed 1 week later by a 500-mg dose. Blood samples were obtained at 0, 15, 30, 45, 60, 90, 120, 180, 240, and 360 min after the injection. Urine samples were collected immediately before injection and at 0 to 2, 2 to 4, 4 to 6, 6 to 12, and 12 to 24 h after the drug had been administered. Blood samples were allowed to clot at room temperature and centrifuged, and the serum was decanted within 1 h of collection. Each serum and urine sample was divided in aliquots, immediately frozen, and stored at -20°C until assay. Cefotaxime in urine, serum, and phosphate buffer was found to be stable for 3 months at -20°C at concentrations of 50 ,ug/ml.Intravenous infusion study. Ten healthy males were selected for this study. Each received 1,000 mg of cefotaxime infused intravenously through a small-bore needle over a 30-min period. Blood samples were drawn before infusion and at 30, 45, 60, 75, 90, 120, 180, and 240 min after the start of the infusion. Urine samples were collected before infusion of the agent and at intervals of 0 to 2, 2 to 4, 4 to 6, and 12 to 24 h. Samples were processed as detailed above.Assays. Cefotaxime was assayed by the agar well diffusion technique, using antibiotic medium no. 2 (Difco Laboratories) as previously described (2,

The kinetics of cefotaxime, a cephalosporin with an unusually broad antibacterial spectrum, were examined in humans after intravenous bolus injection, intravenous infusion every 6 hr for 14 days, and intramuscular injection every 8 hr for 10 days. Mean peak serum level after bolus injection of 500 mg was 37.9 microgram/ml; after 1 gm, 102.4 microgram/ml; and after 2 gm, 214.1 microgram/ml. The half-life (t1/2) was 1 hr for the 3 doses. Total serum clearance was the same for all doses. Overall excretion was 50% to 60%; part of the drug was excreted as the desacetyl derivative. After multiple intravenous infusion the elimination rate constants and t1/2 were the same on days 1 and 15. Assayable levels were present on all days 5 min before injection of a dose. Multiple intramuscular injections of 500 mg produced serum levels of 9.2 to 11.9 microgram/ml. The t1/2 was 0.93 hr. Mean serum levels at 8 hr ranged from 0.08 to 0.55 microgram/ml. Serum levels produced by intravenous infusion or intramuscular injection were inhibitory for most (90%) aerobic gram-positive and gram-negative organisms susceptible to cefotaxime.

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