Clinical pharmacokinetics of 2'-deoxy-2'-methylidenecytidine (DMDC), a deoxycytidine analogue antineoplastic agent.

This article reviews the clinical pharmacokinetics of a deoxycytidine analogue of cytarabine, 2'-deoxy-2'-methylidenecytidine (DMDC). DMDC belongs to the antimetabolite class of anticancer drugs and is phosphorylated into its active, triphosphate, form within the tumour cell. Cancer cell death appears to be a result of the impairment of DNA synthesis by the triphosphate form. DMDC undergoes deamination to the inactive 2'-deoxy-2'-methylideneuridine (DMDU), its main plasma metabolite. Following intravenous administration at 30 to 450 mg/m2, DMDC has low systemic clearance (10 to 15 L/h/m2), moderate volume of distribution (nominally similar to total body water) and a short elimination half-life of between 2 and 6 hours. Renal clearance of DMDC accounts for approximately 30 to 50% of total clearance. Following oral administration of DMDC at 12 to 50 mg/m2, mean maximum DMDC plasma concentrations are within the 100 to 400 microg/L range and are generally reached within 2 hours. Oral bioavailability of DMDC is in the order of 40%, largely as a result of first-pass metabolism in the gut and liver. This first-pass effect results in considerable interpatient variability in systemic exposure to DMDC after oral administration. The systemic availability of DMDC is proportional to the administered dose and, although there was evidence that systemic exposure to DMDC decreased on repeated administration, there are no excessive time-dependent changes in systemic exposure to DMDC. Following oral administration, DMDC is metabolised in the gut wall and liver by deamination to DMDU. The kidneys eliminate DMDC and DMDU, with up to 50% of the administered dose recovered in urine, on average, as parent drug and metabolite. Dose escalation to the maximum tolerated dose was facilitated by a pharmacokinetically guided dose escalation strategy. DMDC has shown activity in non-small-cell lung cancer and colorectal cancers following oral administration. Several tumour responses are observed at the highest doses of DMDC, indicating a possible dose-response relationship with this drug. The main clinical adverse event of DMDC therapy is myelotoxicity. The haematological toxicity of DMDC was schedule dependent; twice daily administration was associated with greater toxic effects than a once daily regimen. A pharmacokinetic-pharmacodynamic model characterised the relationship between plasma DMDC concentrations and the time-dissociated toxicity. This model-dependent approach may be used to predict the consequences of as-yet-untested therapy as well as relating acceptable risks of haematological toxicity to target drug exposure.

Phase I study of ONO-4007, a synthetic analogue of the lipid A moiety of bacterial lipopolysaccharide.

ONO-4007 is a synthetic analogue of the lipid A moiety of bacterial lipopolysaccharide, which exhibits antitumor activity by the induction of intratumoral tumor necrosis factor alpha, the potentiation of tumor-infiltrating macrophages, and the inhibition of angiogenesis. Interleukin (IL)-1 alpha, IL-6, and IL-12 induction by ONO-4007 activates cytotoxic natural killer cells to up-regulate IFN-gamma and nitric oxide synthase activity. ONO-4007 was given to 24 patients (13 males and 11 females; median age, 53 years) as a 30-min i.v. infusion on day 1, followed on day 15 by a first treatment cycle consisting of three weekly infusions at the same dose, followed by a rest period of 1 week. Cohorts of six patients received up to a maximum of four treatment cycles at increasing dose levels (75, 100, and 125 mg). The maximum tolerated dose was 125 mg, with grade 3 National Cancer Institute Common Toxicity Criteria toxicity (rigors with cyanosis) occurring in two of six patients at this dose level. An additional six patients were treated at 100 mg, the dose below the maximum tolerated dose. Other toxicities included grade 2 National Cancer Institute Common Toxicity Criteria myalgia, nausea, and hypotension. The pharmacokinetics of ONO-4007 appeared to be independent of dose and showed linearity with respect to time. ONO-4007 has a low systemic clearance (approximately 1.3 ml/min) and a small volume of distribution (5-8 liters) with a long t1/2 of 74-95 h. The administration of ONO-4007 was shown to result in a significant increase in circulating levels of tumor necrosis factor alpha and IL-6. No objective antitumor responses were observed. Seven patients maintained stable disease for at least two cycles, whereas five patients maintained stable disease for the full four-cycle duration of the study. Additional studies are required to determine the antitumor activity of ONO-4007.

Models of schedule dependent haematological toxicity of 2'-deoxy-2'-methylidenecytidine (DMDC).

OBJECTIVE: DMDC (2'-deoxy-2'-methylidenecytidine) is-a potential antitumour deoxycytidine analogue of cytosine arabinoside. The major dose-limiting toxicity of DMDC is haematological depression, particularly neutropenia, and therefore quantitative exposure-toxicity relationships for DMDC are warranted. METHODS: Data on the survival fraction at nadir of leukocytes, neutrophils and platelets from 66 patients receiving a once-daily regimen and 85 patients receiving a twice-daily regimen of DMDC were related to DMDC concentration-time profiles using area under the plasma concentration-time curve (AUC), threshold and general models. A semiphysiological model of neutrophils versus time after DMDC administration included transient compartments to imitate the differentiation stages in the bone marrow. RESULTS: The relationship between plasma DMDC concentration-time profiles and the haematological toxicity at nadir was best described using an AUC-dependent model with separate functions for once- and twice-daily dosing, indicating schedule dependence of DMDC effects, even if differences in treatment duration had a similar explanatory value. Twice-daily dosing was associated with greater toxic effects than once-daily dosing. The AUC required for a 70% reduction in the neutrophils was 16 mg.h/l and 4.2 mg.h/l for the once- and twice-daily regimens, respectively. The semiphysiological model included nine proliferating transient compartments that were sensitive to DMDC in a schedule-dependent manner, five non-mitotic, non-sensitive compartments and one compartment for circulating neutrophils. CONCLUSIONS: The haematological toxicity of DMDC is schedule dependent. The survival fractions of leukocytes, neutrophils and platelets are predicted to be lower when given on a twice-daily regimen than on a once-daily regimen. A semiphysiological model with transient compartments successfully described the entire time course of neutropenia after DMDC administration.

Metabolic fate of 14C-camostat mesylate in man, rat and dog after intravenous administration.

1. The metabolic fate of N,N-dimethylcarbamoylmethyl 4-(4-guanidino[14C]benzoyloxy)phenylacetate methanesulphonate (14C-camostat mesylate) was investigated after i.v. administration to man (12-h infusion), and to rat and dog (bolus injection). 2. Renal excretion (mainly in 24 h) accounted for at least 80% dose in all three species, and the only two important metabolites were identified as 4-(4-guanidinobenzoyloxy)phenylacetic acid (GBPA) and 4-guanidinobenzoic acid (GBA). 3. Parent drug was not detected in human plasma either during or after infusion of 14C-camostat mesylate owing to rapid hydrolysis of the side-chain ester group (t1/2 < 1 min). Steady-state levels of both GBPA and GBA in plasma were apparently attained by the end of the infusion period. Mean terminal half-life, systemic clearance and apparent volume of distribution at steady-state of GBPA in man were 1.0 h, 6.4 ml/min per kg and 0.38 l/kg, respectively, and the corresponding values for GBA were 2.4 h, 4.7 ml/min per kg and 1.01/kg respectively. 4. Radioactivity was rapidly distributed to most tissues after bolus i.v. doses of 14C-camostat mesylate to rats and dogs, with highest levels being associated with the liver and kidney, the two main organs of drug elimination. Concentrations in the pancreas, a possible site for drug action, were generally lower than those in plasma.

Relationship between pharmacokinetics and pharmacodynamics of tinzaparin (logiparin), a low molecular weight heparin, in dogs.

1. Pharmacodynamic models relating the plasma concentrations (C) of radioactive heparin material to anticoagulant effect (E) have been investigated after single i.v. and s.c. doses of 3H-tinzaparin (1 and 4 mg/kg), a radiolabelled low molecular weight heparin, to six dogs. 2. A counterclockwise hysteresis, characterizing the C versus E relationship, was observed in all animals after s.c., but not i.v., doses indicating a possible delay (lag-time) in the systemic availability of pharmacologically-active heparin material following extravascular administration. A constant (Ke) was introduced into the model to account for this hysteresis. 3. At high plasma concentrations of radioactivity (> 10 micrograms/ml), E was related to C by a sum of two sigmoid Emax models, whereas, at lower concentrations, this reduced to the well-known sigmoid Emax model. It was proposed that tinzaparin activates two 'receptors' having different affinities for the drug. The values of EC50 associated with the activation of a single 'receptor' and of a proposed additional 'receptor' were 3 and 13 micrograms/ml of heparin material, respectively. 4. Heparin material was predominantly eliminated by renal excretion and underwent widespread tissue distribution. After s.c. administration, input of heparin material into systemic plasma was complete within 12 h post-dose, and the absorption process was characterized by a bi-exponential function. 5. We conclude that sigmoid Emax models adequately describe the C versus E relationship after s.c. and i.v. doses of 3H-tinzaparin in dogs and that the interindividual variation of the pharmacodynamic parameters derived from this model was relatively small.

Percutaneous absorption of co-administered N-methyl-2-[14C]pyrrolidinone and 2-[14C]pyrrolidinone in the rat.

The percutaneous absorption has been investigated in rats of a mixture (3:2, w/w) of N-methyl-2-pyrrolidinone (NMP) and 2-pyrrolidinone (2-P), a combination intended for use as a vehicle in the formulation of an antimycotic drug to enhance skin penetration on dermal application, following co-administration of the two 14C-radiolabelled compounds by the dermal and oral routes. Radioactivity was excreted predominantly in the urine after either route of administration, and comparison of the respective excretion profiles indicated that about three-quarters of the applied dose was absorbed through the skin. Plasma concentrations of each parent compound, as determined by radio-HPLC, reached peak values at 2 hr after oral dosing, and remained relatively uniform during 1-6 hr after application to the skin, suggesting constant percutaneous absorption during this period. NMP appeared to be absorbed through the skin more extensively and at a slightly faster rate than 2-P; total percutaneous absorption tended to be more extensive in female than in male rats. Together, these two 14C-compounds accounted for most of the plasma radioactivity up to 6-8 hr post-administration. However, by 12 hr (when plasma levels were relatively low), most of the radioactivity was associated with unknown polar metabolites. In view of the extensive percutaneous absorption and little first-pass metabolism of the two pyrrolidinones, the oral route was considered to represent a valid alternative to the dermal route for the assessment of the systemic toxicity of the two compounds.

Dose-proportional pharmacokinetics of cefepime in rats.

Cefepime (BMY-28142) is a new parenteral cephalosporin antibiotic with excellent activity against a broad-spectrum of clinically important pathogens resistant to other new cephalosporins. A single bolus dose of 10, 20 or 40 mg/kg cefepime was given i.v. to male and female Sprague-Dawley rats from which blood and urine samples were collected. For statistical reasons, pharmacokinetic parameters (AUC, Kel) were derived by fitting an exponential curve to the plasma concentrations; subsequently, contrasts were made between the different doses for AUC and Kel and, in addition, plasma concentrations observed after the first sampling time (Cmax). Cmax and AUC appeared to be linearly related to the administered dose in both males and females. The dose increased in the ratio 1:2:4 and mean Cmax in male and female rats increased in the ratio 1:2.3:4.1 and 1:2.3:4.4 respectively; similarly, AUC increased in the ratio 1:2.2:4.3 and 1:2.0:4.2 respectively. Deviations from linearity and proportionality were not significant (P0.05). The systemic clearance of cefepime in rats was 2.3 ml/min. The volume of distribution was about 60 ml and cefepime appears to be selectively distributed into the extracellular water. Plasma concentrations declined monoexponentially with a mean half-life of 15-20 min which did not significantly change with increasing doses. The renal clearance of cefepime was 1.8 ml/min and approximately 80% of the dose was excreted in the urine unchanged; renal excretion of cefepime is the major route of elimination in rats. The elimination and distribution characteristics of cefepime in rats were similar to those observed in man.

Pharmacokinetics of acitretin and its 13-cis metabolite in patients on haemodialysis.

1. Plasma concentrations of acitretin and its metabolite (13-cis acitretin) were measured in six patients on haemodialysis and six subjects without renal failure following a single oral dose of 50 mg of acitretin. 2. The mean areas under the plasma concentration vs time curves of acitretin and its metabolite were about 50% lower in patients on haemodialysis. 3. No retinoids were detectable in the dialysate.

Overview of recent clinical pharmacokinetic studies with acitretin (Ro 10-1670, etretin).

Following oral administration of acitretin with food, peak plasma concentrations of unchanged drug (Ro 10-1670) are reached within 4 h. The mean absolute bioavailability of acitretin is 59% with high interpatient variability consistent with that of etretinate. Taking acitretin with food results in an increased and more consistent bioavailability. The drug appears to be extensively distributed throughout the body without unexpected accumulation and the elimination half-life is approximately 50 h. Acitretin has a profound pharmacokinetic advantage over etretinate because it is eliminated more rapidly from the body; etretinate is sequestered into fatty tissue due to its greater lipophilicity creating a deep compartment from which it is only slowly released. Acitretin is eliminated entirely by the metabolism and the resultant metabolites are excreted via the kidney and bile.

Food increases the bioavailability of acitretin.

Eighteen healthy male volunteers received 50 mg oral doses of acitretin on two occasions, according to a random crossover design. Acitretin was administered during a complete fast or following a moderate breakfast. Plasma samples were obtained at various times and the concentration of acitretin and its 13-cis isomeric metabolite (Ro 13-7652) were quantified by a specific HPLC assay. The AUC0-15 for acitretin was increased when administered with food for all subjects (except one) with a mean increase of 90% (from 1175 to 2249 ng/ml.hr). The maximum plasma concentration of acitretin (Cmax) was increased by 70% when administered with food (from 245 to 416 ng/ml), while the time to reach Cmax was unaffected. The ratio of AUC of Ro 13-7652 to acitretin was the same for both the fasted and fed conditions; therefore, the formation of metabolite was not influenced by concomitant ingestion of food. The presence of food increases the apparent bioavailability of acitretin. A likely mechanism behind this observation is an increase in acitretin solubility in addition to an increase in the lymphatic absorption and a prolonged residence time of the drug in the gastrointestinal tract.

Cis-trans interconversion of acitretin in man.

The major plasma metabolite of acitretin (trans-acitretin) is its 13-cis isomer, cis-acitretin. Interconversion of cis-acitretin to trans-acitretin was demonstrated in man following administration of a single oral dose of cis-acitretin. Plasma concentrations of Ro 13-7652 (cis-acitretin) and Ro 10-1670 (trans-acitretin) were much higher after cis-acitretin administration than after trans-acitretin administration. Surprisingly, these high concentrations were not associated with a clear therapeutic effect in dermatoses (e.g. psoriasis) which are usually responsive to oral retinoids. Interactions between the cis and trans isomers formed in vivo may explain the difference in therapeutic activity of each stereoisomer when administered orally.

Activity and distribution studies of etoposide and mitozolomide in vivo and in vitro against human choriocarcinoma cell lines.

The in vivo antitumor activity of etoposide and mitozolomide was assessed in nude mice bearing a xenograft (CC3) of human gestational choriocarcinoma. Both agents demonstrated, at best, marginal activity observed as a delay in tumour growth. This lack of sensitivity suggests that the CC3 xenograft is not a good model for selection of agents for clinical evaluation in gestational choriocarcinoma. Plasma and tissue concentrations of etoposide and mitozolomide were measured in nude mice. Drug concentrations found in tumour tissue were 60% and 30% of plasma levels for mitozolomide and etoposide respectively. Etoposide and mitozolomide activity was also evaluated in vitro with another choriocarcinoma cell line (JAR). Maximum cell-kill was achieved after exposure to etoposide 0.05-1 microgram/ml for 3-24 h. In vitro response to etoposide demonstrates the importance of exposure time in determining cytotoxicity. In contrast, mitozolomide at concentrations from 1-100 micrograms/ml did not have a marked effect against JAR after exposure for 3-24 h.

Plasma and tissue disposition of mitozolomide in mice.

When mitozolomide was administered i.p. to mice, drug disposition appeared to fit a simple, one-compartment kinetic model with an elimination half-life of less than 1 h. The disposition of mitozolomide in mice bearing the ROS osteosarcoma, also followed a first-order process but in this case the elimination of the drug was significantly faster from plasma, liver, lung and kidney tissue compared to the elimination half-life of the drug from the same tissues of mice without tumour (P less than 0.05). Mitozolomide was rapidly and extensively distributed into tissues, including the tumour. Mitozolomide was not concentrated in any particular tissue although the brain contained the lowest drug concentration compared to any tissue studied. After 4 h following administration, mitozolomide could not be measured in plasma or tissues. AUC values calculated from mitozolomide concentration versus time profiles in plasma, liver and kidney homogenates were 27-29% lower in mice pretreated with phenobarbitone compared to those values obtained from mice administered saline only, (P less than 0.02). Since phenobarbitone is known to induce liver microsomal enzymes, it is possible that hepatic metabolism is involved in the degredation of mitozolomide.

Phase I clinical trial of mitozolomide.

Mitozolomide (NSC-353451; CCRG 81010; M and B 39565) is a novel potential anticancer agent that was selected for phase I study on the basis of broad spectrum activity in mouse tumors. Initially, mitozolomide was given iv as a short infusion to 37 patients in doses ranging from 8 to 153 mg/m2. Nausea and vomiting was dose-related but was not severe. The dose-limiting toxic effect was thrombocytopenia at doses greater than 115 mg/m2, and recovery from the thrombocytopenia was delayed up to 8 weeks. Partial responses were seen in two patients with adenocarcinoma of the ovary. The pharmacokinetics of mitozolomide showed that the half-life of the intact drug in the plasma was between 1 and 1.3 hours. The area under the curve was proportional to the dose administered. Mitozolomide is well-absorbed; therefore, future studies are recommended using a single-dose schedule orally or iv. In the phase I study reported here, a dose of 115 mg/m2 appeared to be safe, but additional studies have shown that when given orally to an older population, most patients experienced thrombocytopenia less than 50,000 cells/mm3. The recommended dose using current data is 90 mg/m2 iv or orally.

Pharmacokinetics and toxicity of the epipodophyllotoxin derivative etoposide (VP 16-213) in patients with gestational choriocarcinoma and malignant teratoma.

Serum levels of etoposide obtained 5 min after administration of 100 mg/m2 were between 11 and 30 micrograms/ml. By 24 h after drug administration, serum levels had fallen to between 0.19 and 1.11 micrograms/ml. Interpatient variation of etoposide serum concentrations obtained 5 min after drug administration was low, whereas interpatient variation 24 h later was noticeably higher. A significant correlation was observed (r = -0.698) between the WBC nadir and the mean etoposide serum concentrations, measured 24 h after drug administration, in patients receiving etoposide in combination with cyclophosphamide and actinomycin D. However, a relationship was not observed in those patients receiving etoposide alone. There was no observed difference in the efficacy or toxicity of 500 mg/m2 etoposide when the dose was administered either as 100 mg/m2 on each of 5 consecutive days or as 250 mg/m2 on days 1 and 3. There was no significant difference between AUC values calculated from etoposide concentration versus time profiles in patients receiving the drug on days 1 and 3 and those values obtained with the 5-day schedule. Patients resistant to a conventional dose of etoposide were given a higher dose of 1 g/m2/24 h, but this schedule did not cause an increase in efficacy despite an increase in serum levels of the drug. CSF levels in two of these patients receiving high-dose etoposide were 1.28% and 2.09% of the serum concentrations.