Mass balance and metabolite profiling of 14C-guadecitabine in patients with advanced cancer.

Purpose The objective of this mass balance trial was to determine the excretory pathways and metabolic profile of the novel anticancer agent guadecitabine in humans after administration of a 14C-radiolabeled dose of guadecitabine. Experimental design Included patients received at least one cycle of 45 mg/m2 guadecitabine subcutaneously as once-daily doses on Days 1 to 5 of a 28-day cycle, of which the 5th (last) dose in the first cycle was spiked with 14C-radiolabeled guadecitabine. Using different mass spectrometric techniques in combination with off-line liquid scintillation counting, the exposure and excretion of 14C-guadecitabine and metabolites in the systemic circulation, excreta, and intracellular target site were established. Results Five patients were enrolled in the mass balance trial. 14C-guadecitabine radioactivity was rapidly and almost exclusively excreted in urine, with an average amount of radioactivity recovered of 90.2%. After uptake in the systemic circulation, guadecitabine was converted into ß-decitabine (active anomer), and from ß-decitabine into the presumably inactive metabolites M1-M5. All identified metabolites in plasma and urine were ß-decitabine related products, suggesting almost complete conversion via cleavage of the phosphodiester bond between ß-decitabine and deoxyguanosine prior to further elimination. ß-decitabine enters the intracellular activation pathway, leading to detectable ß-decitabine-triphosphate and DNA incorporated ß-decitabine levels in peripheral blood mononuclear cells, providing confirmation that the drug reaches its DNA target site. Conclusion The metabolic and excretory pathways of guadecitabine and its metabolites were successfully characterized after subcutaneous guadecitabine administration in cancer patients. These data support the clinical evaluation of safety and efficacy of the subcutaneous guadecitabine drug product.

Development and validation of LC-MS/MS methods for the quantification of the novel anticancer agent guadecitabine and its active metabolite ß­decitabine in human plasma, whole blood and urine.

Guadecitabine (SGI-110), a dinucleotide of ߭decitabine and deoxyguanosine, is currently being evaluated in phase II/III clinical trials for the treatment of hematological malignancies and solid tumors. This article describes the development and validation of bioanalytical assays to quantify guadecitabine and its active metabolite ߭decitabine in human plasma, whole blood and urine using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Since ߭decitabine is rapidly metabolized further by cytidine deaminase, plasma and whole blood samples were kept on ice-water after collection and stabilized with tetrahydrouridine (THU) directly upon sample collection. Sample preparation consisted of protein precipitation for plasma and whole blood and dilution for urine samples and was further optimized for each matrix and analyte separately. Final extracts were injected onto a C6-phenyl column for guadecitabine analysis, or a Nova-Pak Silica column for ߭decitabine analysis. Gradient elution was applied for both analytes using the same eluents for each assay and detection was performed on triple quadrupole mass spectrometers operating in the positive ion mode (Sciex QTRAP 5500 and QTRAP 6500). The assay for guadecitabine was linear over a range of 1.0-200 ng/mL (plasma, whole blood) and 10-2000 ng/mL (urine). For ߭decitabine the assay was linear over a range of 0.5-100 ng/mL (plasma, whole blood) and 5-1000 ng/mL (urine). The presented methods were successfully validated according to the latest FDA and EMA guidelines for bioanalytical method validation and applied in a guadecitabine clinical mass balance trial in patients with advanced cancer.

Enzymatic activation of double-targeted 5'-O-L-valyl-decitabine prodrug by biphenyl hydrolase-like protein and its molecular design basis.

A primary focus of this research was to explore the activation process and mechanism of decitabine (5-aza-2'-deoxycytidine, DAC) prodrug. Recently, it has been reported that biphenyl hydrolase-like protein (BPHL) can play an important role in the activation of some amino acid nucleoside prodrugs with a general preference for hydrophobic amino acids and 5'-esters. Therefore, we put forward a bold hypothesis that this novel enzyme may be primarily responsible for the activation process of DAC prodrug as well. 5'-O-L-valyl-decitabine (L-val-DAC) was synthesized before and can be transported across biological membranes by the oligopeptide transporter (PEPT1), granting it much greater utility in vivo. In this report, L-val-DAC was found to be a good substrate of BPHL protein (K m 0.59 mM; k cat/K m 553.69 mM-1 s-1). After intestinal absorption, L-val-DAC was rapidly and almost completely hydrolyzed to DAC and L-valine. The catalysis was mainly mediated by the BPHL hydrolase and resulted in the intestinal first-pass effect of L-val-DAC after oral administration in Sprague-Dawley rats with cannulated jugular and portal veins. The structural insights using computational molecular docking showed that BPHL had a unique binding mode for L-val-DAC. As a fundamental basis, the simulation was employed to explain the catalytic mechanism in molecular level. In conclusion, BPHL was at least one of the primary candidate enzymes for L-val-DAC prodrug activation. This promising double-targeted prodrug approach have more advantages than the traditional targeted designs due to its higher transport and more predictable activation, thereby leading to a favorable property for oral delivery.

A robust and rapid liquid chromatography tandem mass spectrometric method for the quantitative analysis of 5-azacytidine.

The DNA methyltransferase inhibitor 5-azacytidine is being evaluated clinically as an oral formulation to treat various solid tumors. A sensitive, reliable method was developed to quantitate 5-azacytidine using LC-MS/MS to perform detailed pharmacokinetic studies. The drug of interest was extracted from plasma using Oasis MCX ion exchange solid-phase extraction 96-well plates. Chromatographic separation was achieved with a YMC J'sphere M80 C18 column and isocratic elution with a methanol-water-formic acid (15:85:0.1, v/v/v) mobile phase over a 7 min total analytical run time. An AB Sciex 5500 triple quadrupole mass spectrometer operated in positive electrospray ionization mode was used for the detection of 5-azacytidine. The assay range was 5-500 ng/mL and proved to be accurate (97.8-109.1%) and precise (CV ≤ 9.8%). Tetrahydrouridine was used to stabilize 5-azacytidine in blood/plasma samples. With the addition of tetrahydrouridine, long-term frozen plasma stability for 5-azacytidine at -70°C has been determined for at least 323 days. The method was applied for the measurement of total plasma concentrations of 5-azacytidine in a cancer patient receiving a 300 mg oral daily dose.

Pharmacokinetics and Pharmacodynamics with Extended Dosing of CC-486 in Patients with Hematologic Malignancies.

UNLABELLED: CC-486 (oral azacitidine) is an epigenetic modifier in development for patients with myelodysplastic syndromes and acute myeloid leukemia. In part 1 of this two-part study, a 7-day CC-486 dosing schedule showed clinical activity, was generally well tolerated, and reduced DNA methylation. Extending dosing of CC-486 beyond 7 days would increase duration of azacitidine exposure. We hypothesized that extended dosing would therefore provide more sustained epigenetic activity. Reported here are the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of CC-486 extended dosing schedules in patients with myelodysplastic syndromes (MDS), chronic myelomonocytic leukemia (CMML) or acute myeloid leukemia (AML) from part 2 of this study. PK and/or PD data were available for 59 patients who were sequentially assigned to 1 of 4 extended CC-486 dosing schedules: 300mg once-daily or 200mg twice-daily for 14 or 21 days per 28-day cycle. Both 300mg once-daily schedules and the 200mg twice-daily 21-day schedule significantly (all P < .05) reduced global DNA methylation in whole blood at all measured time points (days 15, 22, and 28 of the treatment cycle), with sustained hypomethylation at cycle end compared with baseline. CC-486 exposures and reduced DNA methylation were significantly correlated. Patients who had a hematologic response had significantly greater methylation reductions than non-responding patients. These data demonstrate that extended dosing of CC-486 sustains epigenetic effects through the treatment cycle. TRIAL REGISTRATION: ClinicalTrials.gov NCT00528983.

Development and validation of a HILIC-MS/MS method for quantification of decitabine in human plasma by using lithium adduct detection.

A highly sensitive, selective, and rugged quantification method was developed and validated for decitabine (5-aza-2'-deoxycytidine) in human plasma treated with 100µg/mL of tetrahydrouridine (THU). Chromatographic separation was accomplished using hydrophilic interaction liquid chromatography (HILIC) and detection used electrospray ionization (ESI) tandem mass spectrometry (MS/MS) by monitoring lithiated adducts of the analytes as precursor ions. The method involves simple acetonitrile precipitation steps (in an ice bath) followed by injection of the supernatant onto a Thermo Betasil Silica-100, 100×3.0mm, 5µm LC column. Protonated ([M+H](+)), sodiated ([M+Na](+)), and lithiated ([M+Li](+)) adducts as precursor ions for MS/MS detection were evaluated for best sensitivity and assay performance. During initial method development abundant sodium [M+Na](+) and potassium [M+K](+) adducts were observed while the protonated species [M+H](+) was present at a relative abundance of less than 5% in Q1. The alkali adducts were not be able to be minimized by the usual approach of increasing acid content in mobile phases. Significant analyte/internal standard (IS) co-suppression and inter-lot response differences were observed when using the sodium adduct as the precursor ion for quantification. By adding 2mM lithium acetate in aqueous mobile phase component, the lithium adduct effectively replaced other cationic species and was successfully used as the precursor ion for selected reaction monitoring (SRM) detection. The method demonstrated the separation of anomers and from other endogenous interferences using a 3-min gradient elution. Decitabine stock, working solution stabilities were investigated during method development. Three different peaks, including one from anomerization, were observed in the SRM transition of the analyte when it was in neutral aqueous solution. The assay was validated over a concentration range of 0.5-500ng/mL (or 0.44-440pg injected on column) in 50µL of human plasma. The accuracy and precision were within 8.6% relative error and 6.3% coefficient of variation, respectively. Decitabine was stable in THU treated human plasma for at least 68 days and after 5 freeze-thaw cycles when stored at -70°C. Stability of decitabine in THU treated human whole blood, matrix factor and recovery were also evaluated during method validation. The method was successfully used for clinical sample analysis.

Pharmacokinetics of different formulations of oral azacitidine (CC-486) and the effect of food and modified gastric pH on pharmacokinetics in subjects with hematologic malignancies.

Parenteral azacitidine improves overall survival in higher-risk myelodysplastic syndromes. An oral azacitidine formulation would allow extended dosing schedules, potentially improving safety and/or efficacy. Two Phase 1 studies evaluated the pharmacokinetics (PK) of oral azacitidine in subjects with hematologic malignancies. Study 1 evaluated different oral formulations (immediate release tablet [IRT], enteric-coated tablet, and capsule; N = 16). Study 2 assessed the effect of food (Part 1; N = 17) and gastric pH modulation with omeprazole (Part 2; N = 14) on oral azacitidine PK. Azacitidine plasma concentration-time profiles for IRT and capsule formulations were similar, with more rapid time to maximum plasma concentration (Tmax ) than the enteric-coated tablet. Study 2 evaluated only IRT formulations of oral azacitidine. Under fed condition, Tmax was delayed ∼1.5 hours but area under the concentration-time curve (AUC∞ ) and maximum plasma concentrations (Cmax ) were comparable under fed and fasted conditions. Mean azacitidine AUC∞ and Cmax increased upon omeprazole co-administration (18.3% and 13.2%, respectively, vs. oral azacitidine alone), but not to a clinically meaningful extent. High inter-subject variability in AUC∞ and Cmax (%CV range 46.4-68.9%) was observed. Oral azacitidine is rapidly absorbed with little or no effect of food on PK parameters, and does not require dose adjustments when taking a proton-pump inhibitor.

An HPLC-MS/MS method for simultaneous determination of decitabine and its valyl prodrug valdecitabine in rat plasma.

A simple and sensitive HPLC-MS/MS method was developed and validated for the simultaneous determination of decitabine and valdecitabine in rat plasma. The analytes were separated on a C(18) column (150mm×4.6mm, 3.5µm) and a triple-quadrupole mass spectrometer equipped with an electrospray ionization (ESI) source was applied for detection. A clean solid-phase extraction procedure with cation exchange cartridge was employed to extract the analytes from rat plasma with high recovery of decitabine (>82%). The calibration curves were linear over a concentration range of 10-10,000ng/mL for decitabine and 5-500ng/mL for valdecitabine. The lower limit of quantitation (LLOQ) of decitabine and valdecitabine was 10 and 5ng/mL, respectively. The intra-day and inter-day precisions were less than 15% and the relative error (RE) was all within ±15%. The validated method was successfully applied to a pharmacokinetics study in rats after either decitabine or valdecitabine orally administrated to the Sprague-Dawley rats.

Development and validation of a liquid chromatography-tandem mass spectrometry method for quantification of decitabine in rat plasma.

Decitabine is chemically unstable at physiological temperature and pH. In addition, the bioanalysis of decitabine is easily interfered by endogenous 2-deoxycytidine. A simple, sensitive and specific LC-MS/MS method was developed for the analysis of decitabine in rat plasma. No exogenous stabilizers were used to prevent the degradation of decitabine in rat plasma. After deproteinized with acetonitrile at room temperature, rat plasma samples were analyzed on a Hypersil APS-2 NH2 column interfaced with a triple quadrupole tandem mass spectrometer in positive electrospray ionization mode. Decitabine was completely separated from 2-deoxycytidine using gradient elution of water (solvent A) and acetonitrile (solvent B) at a flow rate of 1 mL/min. To quantify decitabine and daidzin (internal standard), respectively, multiple reaction monitoring (MRM) transitions of m/z 251.1→134.7 and m/z 417.3→255.3 was performed. The assay was linear over the concentration range of 5.0-2000 ng/mL. The intra- and inter-day precision was within 12.0% in terms of relative standard deviation (RSD%) and the accuracy within 5.9% in terms of relative error. The LC-MS/MS method was fully validated for its sensitivity, selectivity, stability study, matrix effect and recovery. The data indicate that this LC-MS/MS method is a specific and effective method for the pharmacokinetic study of decitabine in rat plasma. Compared with the previously reported analytical methods, this method showed easy and economic sample preparation, good specificity and high sensitivity with less plasma (50 µL).

Phase I trial of low dose decitabine targeting DNA hypermethylation in patients with chronic lymphocytic leukaemia and non-Hodgkin lymphoma: dose-limiting myelosuppression without evidence of DNA hypomethylation.

Targeting aberrant DNA hypermethylation in chronic lymphocytic leukaemia (CLL) and non-Hodgkin lymphoma (NHL) with decitabine may reverse epigenetic silencing in B-cell malignancies. Twenty patients were enrolled in two phase I trials to determine the minimum effective pharmacological dose of decitabine in patients with relapsed/refractory CLL (n = 16) and NHL (n = 4). Patients received 1-3 cycles of decitabine. Dose-limiting toxicity (DLT) was observed in 2 of 4 CLL and 2 of 2 NHL patients receiving decitabine at 15 mg/m(2) per d days 1-10, consisting of grade 3-4 thrombocytopenia and hyperbilirubinaemia. Six patients with CLL received decitabine at 10 mg/m(2) per d days 1-10 without DLT; however, re-expression of methylated genes or changes in global DNA methylation were not observed. Therefore, a 5-day decitabine schedule was examined. With 15 mg/m(2) per d decitabine days 1-5, DLT occurred in 2 of 6 CLL and 2 of 2 NHL patients, consisting of grade 3-4 neutropenia, thrombocytopenia, and febrile neutropenia. Eight patients had stable disease. In 17 patients, there were no significant changes in genome-wide methylation or in target gene re-expression. In conclusion, dose-limiting myelosuppression and infectious complications prevented dose escalation of decitabine to levels associated with changes in global methylation or gene re-expression in CLL and NHL.

Simultaneous determination of decitabine and vorinostat (Suberoylanalide hydroxamic acid, SAHA) by liquid chromatography tandem mass spectrometry for clinical studies.

A reverse-phase high-performance liquid chromatography method with electrospray ionization and detection by tandem mass spectrometry is described for the simultaneous quantitative determination of decitabine (5-aza-2'-deoxycytidine) and vorinostat (Suberoylanalide hydroxamic acid, SAHA) in human plasma. The method involves a simple acetonitrile precipitation step and centrifugation followed by injection of the supernatant onto a C18 150mmx2.1mm I.D., 3microm HPLC column at 36 degrees C. Separation of decitabine, SAHA and their respective internal standards was achieved with a gradient elution and detection was via the mass spectrometer operated in selected reaction monitoring mode. The method was within the defined validation parameters for linearity, repeatability, reproducibility and stability. The limit of detection was determined as 1.0 and 0.125ngml(-1) and lower limits of quantitation were 10 and 1ngml(-1) for decitabine and SAHA, respectively. Effects of sample preparation on stability were also evaluated in human plasma. For clinical sample handling tetrahydrouridine, an inhibitor of cytidine deaminase was found to help prevent decitabine degradation. The method is currently being used in clinical pharmacokinetic studies for the evaluation of decitabine and SAHA combination therapies.

Azacitidine pharmacokinetics in an adolescent patient with renal compromise.

Azacitidine pharmacokinetic parameters in adolescent patients with renal compromise are not available from the medical literature. We describe a 14-year-old with myelodysplastic syndrome treated with subcutaneous 5-azacitidine for disease relapse 2 years after hematopoietic stem cell transplant. Because of renal compromise, malnutrition, and poor functional status, pharmacokinetic parameters were projected from existing literature data to select the patient's first azacitidine treatment course (1.5 mg/kg/d for 7 d). Posttreatment azacitidine plasma concentrations used to calculate patient-specific pharmacokinetic parameters corroborated initial estimates and systemic exposure associated with therapeutic benefit in adults and permitted individualization of treatment.

Characterization of decomposition products and preclinical and low dose clinical pharmacokinetics of decitabine (5-aza-2'-deoxycytidine) by a new liquid chromatography/tandem mass spectrometry quantification method.

Aberrant DNA methylation patterns resulting in gene transcriptional repression are observed in numerous cancers. Decitabine, a DNA methyltransferase inhibitor, is being clinically evaluated in patients with hematologic malignancies and solid tumors. Decitabine is rather unstable and decomposes to 1-beta-D-2'-deoxyribofuranosyl-3-guanylurea under basic conditions and several additional unknown products under neutral conditions. This has greatly limited application of pharmacokinetic assays to clinical development of decitabine. In this paper, a high-performance liquid chromatography/ultraviolet multi-stage mass spectrometry (HPLC-UV-MSn) study of the decomposition of decitabine in water and human plasma revealed that these previously unknown products are isomers of the intermediates formyl-1-beta-D-2'-deoxyribofuranosyl-3-guanylurea and 1-beta-D-2'-deoxyribofuranosyl-3-guanylurea. A HPLC tandem mass spectrometry (MS/MS) method for the determination of decitabine concentrations in human and rat plasma has been developed. This method was based on a specific fragmentation pathway of the molecular ion of decitabine at m/z 229 to generate a unique fragment ion at m/z 113 under collision-induced dissociation. Separation of decitabine and the stable internal standard dihydro-5-aza-cytidine from the endogenous interfering substance in plasma extract was carried out on a C18 Aquasil column under an isocratic elution with a mobile phase consisting of 5% water/acetonitrile and 10 mM ammonium formate. The detection of decitabine was via selected reaction monitoring (SRM, 229 > 113), and its ionization was enhanced by post-column addition of acetonitrile. Effects of sample preparation and handling parameters on the stability of decitabine were also evaluated in human plasma at various temperatures. The accuracy and precision of this assay showed a coefficient of variation of <15% over the range of 0.5-25 ng for rat plasma and 0.1-25 ng for human plasma injected on-column. Pharmacokinetics of decitabine in rats following intravenous doses of 1.0 and 5.0 mg/kg were characterized. In the rat, plasma concentration-time profiles were found to follow a biexponential decline and the pharmacokinetics was dose-independent. Application of this decitabine pharmacokinetic assay to human studies is therefore justified and ongoing.

Azacitidine: a novel agent for myelodysplastic syndromes.

PURPOSE: The pharmacology, pharmacokinetics, clinical efficacy, dosage and administration, and safety of azacitidine are reviewed. SUMMARY: Azacitidine is the first drug in a new class of compounds, known as DNA hypomethylating agents, to receive FDA-approved labeling for the treatment of myelodysplastic syndromes. It exerts its antineoplastic activity by causing a direct cytotoxic effect on abnormally proliferating hematopoietic cell lines by interfering with nucleic acid metabolism. Azacitidine is rapidly absorbed following subcutaneous injection, with peak plasma concentrations achieved within 30 minutes of administration. Based on promising results in Phase I-II testing, azacitidine entered Phase III testing in all subtypes of myelodysplastic syndromes. Azacitidine was compared with best supportive care, the previous standard therapy for myelodysplastic syndromes, demonstrating improvements in hematologic response, delaying time to progression to acute myelogenous leukemia, and increasing overall survival. Azacitidine is available as sterile lyophilized powder in single-use vials for reconstitution. The recommended dosage of azacitidine for the first treatment cycle is 75 mg/m(2) daily for seven days. The treatment cycle should be repeated every four weeks for a minimum of four cycles. Overall, azacitidine appears to be well tolerated, with the most common adverse effects being myelosuppression, nausea, and vomiting. CONCLUSION: Azacitidine is the first DNA hypomethylating agent approved by FDA for the treatment of myelodysplastic syndromes and has demonstrated superior efficacy and improvements in patients' quality of life and bone marrow function over supportive care.

Quantification of 5-azacytidine in plasma by electrospray tandem mass spectrometry coupled with high-performance liquid chromatography.

5-Azacytidine (5AC), a nucleoside analogue and hypomethylating agent, has anticancer properties and has been utilized in the treatment of various malignancies. 5AC is unstable and rapidly hydrolyzed to several by-products, including 5-azacytosine and 5-azauracil. A sensitive, reliable method was developed to quantitate 5AC using LC/MS/MS to perform pharmacokinetic and pharmacodynamic studies on 5AC combination therapy trials. Blood samples were collected in a heparinized tube and immediately processed for storage. To increase the stability of 5AC in plasma, 25 ng/mL tetrahydrouridine was added to the plasma and snap frozen. Plasma samples were extracted using acetonitrile then cleaned up by Oasis MCX ion exchange solid-phase extraction cartridges. 5AC was separated on an YMC Jsphr M80 C(18) column with gradient elution of ammonium acetate (2 mM) with 0.1% formic acid and methanol mobile phase. 5AC elutes at 5.0 +/- 0.2 min with a total run time of 30 min. Identification was through positive-ion mode and multiple reaction monitoring mode at m/z+ 244.9-->113.0 for 5AC and m/z+ 242.0-->126.0 for 5-methyl-2'-deoxycytidine, the internal standard. The lower limit of quantitation of 5AC was 5 ng/mL in human plasma, and linearity was observed from 5 to 500 ng/mL fitted by linear regression with 1/x weight. This method is 50 times more sensitive than previously published assays and successfully allows studies to characterize the pharmacokinetics and pharmacodynamics of 5AC.

Pilot phase I-II study on 5-aza-2'-deoxycytidine (Decitabine) in patients with metastatic lung cancer.

5-Aza-2'-deoxycytidine (5-AZA-CdR, Decitabine) is a nucleoside analog and an active drug for the therapy of acute leukemia. The incorporation of 5-AZA-CdR into DNA blocks DNA methylation and can result in the activation of specific genes, such as tumor suppressor genes. This novel mechanism of action of 5-AZA-CdR stimulated our interest in its potential for cancer therapy in patients with lung cancer. Using a colony assay we observed that 5-AZA-CdR showed a potent antineoplastic effect against two human lung carcinoma cell lines. The objective of this preliminary phase I-II study was to evaluate the toxicity and clinical efficacy of 5-AZA-CdR in patients with stage IV non-small cell lung carcinoma. There were 15 patients that entered the clinical study. For nine assessable patients that received 5-AZA-CdR by a single 8 h i.v. infusion of 200-660 mg/m2 for one or more cycles, the median survival duration was 6.7 months, with three patients surviving more than 15 months. The steady-state plasma concentration of 5-AZA-CdR during the infusion was estimated in some patients and was in the same range that produced activation of a tumor suppressor gene in human lung tumor cell lines as reported by other investigators. The major side effect of 5-AZA-CdR was hematopoietic toxicity which required a 5-6 week recovery period before the next cycle of therapy. This study suggests that 5-AZA-CdR may have some clinical activity against metastatic lung carcinoma using this type of dose schedule.

Preclinical pharmacology of arabinosyl-5-azacytidine in nonhuman primates.

The plasma and cerebrospinal fluid (CSF) pharmacokinetics of arabinosyl-5-azacytidine (AAC) were studied in rhesus monkeys following a 15-min, 1-h, or 12-h i.v. infusion of 200 mg/kg. No clinically significant toxicity was observed with these schedules. The plasma elimination of AAC is rapid and characterized by a triphasic decay with t1/2 alpha = 3.6-5.4 min, t1/2 beta = 18-24 min, and t1/2 gamma = 94-144 min for the above infusion schedules. The CSF penetration of AAC as measured by the CSF:plasma Css ratio for the 12-h infusion was 0.15. The stability of AAC in pooled plasma, phosphate buffered saline, and RPMI 1640 culture media at 37 degrees C was compared with the terminal half-life of AAC observed in vivo. The shorter in vitro AAC half-life in plasma with or without tetrahydrouridine versus that in phosphate buffered saline suggests that the terminal half-life of AAC in vivo is most likely a result of enhanced nucleophilic attack and hydrolytic degradation of the unstable triazine ring in plasma. A triexponential equation modeling the disappearance of AAC was constructed from the in vivo experimental data. Use of this equation in computer-aided simulations of current Phase I doses and schedules of AAC correctly predicts the human plasma concentrations which have been observed. The preclinical pharmacokinetic data provided here may be useful in helping to develop rational human studies with specific concentration x time goals.

[Pharmacokinetic characteristics of 6-azacytidine]. / Osobennosti farmakokinetiki 6-azatsitidina.

Pharmacokinetics of 6-azacytidine, an antitumour drug, is studied by ion-pair reversed-phase highly effective liquid chromatography. The survey of the drug elimination from blood is of a two-phase character: the first phase--with a biological half-life of 5.5 min (for rats) and 6.5 min (for rabbits); the second phase--1.2 and 2.5 h, respectively. For a 24 h period 75% of the drug is excreted unchanged in the urine.