tion of DNA methylation in vitro does not cause major suppression of DNA synthesis. Hypomethylation may restore normal function to genes that are critical for differentiation and proliferation. The cytotoxic effects of azacitidine cause the death of rapidly dividing cells, including cancer cells that are no longer responsive to normal growth control mechanisms. Non-proliferating cells are relatively insensitive to azacitidine.
12.3 Pharmacokinetics
The pharmacokinetics of azacitidine were studied in 6 MDS patients following a single 75mg/m2 subcutaneous (SC) dose and a single 75 mg/m2 intravenous (IV) dose. Azacitidine is rapidly absorbed after SC administration; the peak plasma azacitidine concentration of 750±403ng/ml occurred in 0.5 hour. The bioavailability of SC azacitidine relative to IV azacitidine is approximately 89%, based on area under the curve. Mean volume of distribution following IV dosing is 76 ± 26 L. Mean apparent SC clearance is 167 ± 49 L/hour and mean half-life after SC administration is 41 ± 8 minutes.
Published studies indicate that urinary excretion is the primary route of elimination of azacitidine and its metabolites. Following IV administration of radioactive azacitidine to 5 cancer patients, the cumulative urinary excretion was 85% of the radioactive dose. Fecal excretion accounted for <1% of administered radioactivity over 3 days. Mean excretion of radioactivity in urine following SC administration of 14C-azacitidine was 50%. The mean elimination half-lives of total radioactivity (azacitidine and its metabolites) were similar after IV and SC administrations, about 4 hours.
Special Populations
The effects of renal or hepatic impairment, gender, age, or race on the pharmacokinetics of azacitidine have not been studied [see Dosage and Administration (2.4), Contraindications (4.1) and Warnings and Precautions (5.2, 5.3)].
Drug-Drug Interactions
Drug interaction studies with azacitidine have not been conducted.
An in vitro study of azacitidine incubation in human liver fractions indicated that azacitidine may be metabolized by the liver. Whether azacitidine metabolism may be affected by known microsomal enzyme inhibitors or inducers has not been studied.
The potential of azacitidine to inhibit cytochrome P450 (CYP) enzymes is not known.
In vitro studies with human cultured hepatocytes indicate that azacitidine at concentrations of 1.0μM to 100 μM does not induce CYP 1A2, 2C19, or 3A4/5.
13 NONCLINICAL TOXICOLOGY
Carcinogenesis, Mutagenesis, Impairment of Fertility
The potential carcinogenicity of azacitidine was eva luated in mice and rats. Azacitidine induced tumors of the hematopoietic system in female mice at 2.2 mg/kg (6.6 mg/m2, approximately 8% the recommended human daily dose on a mg/m2 basis) administered IP three times per week for 52 weeks. An increased incidence of tumors in the lymphoreticular system, lung, mammary gland, and skin was seen in mice treated with azacitidine IP at 2.0mg/kg (6.0mg/m2, approximately 8% the recommended human daily dose on a mg/m2 basis) once a week for 50weeks. A tumorigenicity study in rats dosed twice weekly at 15 or 60mg/m2 (approximately 20-80% the recommended human daily dose on a mg/m2 basis) revealed an increased incidence of testicular tumors compared with controls.
The mutagenic and clastogenic potential of azacitidine was tested in in vitro bacterial systems Salmo
