Absorption of Etoposide (VP-16-213) from the Small Intestine of the Rat. The Potential Role of Mucus as an Absorption Rate Limiting Barrier

Stability of the i.v. and oral formulations of etoposide in solution

Etoposide is a widely used cytotoxic drug that requires complex formulation for both the i.v. and oral preparation to ensure drug stability. Data on the stability of the i.v. formulation when diluted in infusion fluids are contradictory, and there is little information on the stability of the oral preparation in gastric or intestinal fluids. The stability of both i.v. and oral etoposide was therefore evaluated in the present investigation. The stability of the i.v. preparation was investigated across a range of concentrations in infusion fluids, being determined by regular sampling for high-performance liquid chromatography (HPLC) analysis and by visual inspection. The stability of the oral preparation was studied in both artificial gastric and intestinal fluids, again with regular sampling for HPLC analysis, and the influence of pH, concentration and the addition of ethanol and bile salts on oral stability was determined. The i.v. preparation showed a marked decrease in stability with increasing drug concentration, but stability was additionally reduced in i.v. bags regularly sampled with a syringe and needle as compared with bags that were inspected visually only (minimal stability in sampled bags, 24 h at 0.5 mg/ml and 5 h at 1.0 mg/ml, as compared with 10 days and 18 h at the respective concentrations in unsampled bags). Stability was also greater at room temperature, 20-23 degrees C, as compared with 8-12 degrees C. Loss of stability was indicated by a decrease in etoposide concentration (measured by HPLC) and the appearance of a fine white precipitate, shown to be pure etoposide. Importantly, the appearance of precipitate was as sensitive as a specific HPLC assay in detecting loss of stability and was in many cases apparent when the etoposide concentration was within 5% of the starting concentration. The oral formulation also showed a marked concentration-dependent decrease in stability in artificial intestinal fluid at pH 7.5 (percentage of etoposide in solution after 2 h at 0.5, 1.0, 1.5 and 2.0 mg/ml, 94 +/- 2%, 80 +/- 5%, 68 +/- 13% and 41 +/- 9%, respectively). There was no concentration effect on stability in gastric fluid at pH 3.0, although stability was much greater at pH 3 and pH 5 as compared with pH 1 or in intestinal fluid at pH 7.5.(ABSTRACT TRUNCATED AT 250 WORDS)

The clinical pharmacology of etoposide and teniposide

Etoposide and teniposide are semisynthetic derivatives of podophyllotoxin and are increasingly used in cancer medicine. Teniposide is more highly protein-bound than etoposide, and its uptake and binding to cells is also greater. Etoposide and teniposide are phase-specific cytotoxic drugs acting in the late S and early G2 phases of the cell cycle. They appear to act by causing breaks in DNA via an interaction with DNA topoisomerase II or by the formation of free radicals. Teniposide is more potent as regards the production of DNA damage and cytotoxicity. Most studies show a biexponential decay following intravenous administration of etoposide and teniposide. The terminal elimination half-life of etoposide is less than that of teniposide, and the plasma and renal clearances of etoposide are greater. The peak plasma concentrations of drug and the area under the concentration versus time curve are linearly related to the intravenous dose of both drugs. Considerable interpatient variability of pharmacokinetic parameters exists following intravenous etoposide and teniposide. Various metabolites of etoposide and teniposide have been identified but their detection and quantitation are disputed. Approximately 30 to 70% of a dose of etoposide is accounted for by excretion, whereas the figure appears to be only 5 to 20% for teniposide. The bioavailability of oral etoposide is about 50% but its absorption is not linear with increasing dose within the range in clinical use. There is considerable inter- and intrapatient variability in the pharmacokinetics of oral etoposide. There is no evidence of accumulation of etoposide and teniposide after multiple consecutive doses by the intravenous or oral routes. The exact roles of the liver and kidney in metabolism and excretion of etoposide and teniposide are uncertain. Etoposide has been shown to be a highly schedule-dependent drug in clinical studies. This together with the phase-specific action of etoposide and teniposide and their increasingly widespread use in cancer medicine make the clinical pharmacology of these drugs of great clinical importance.