D. Dale Shoemaker

Echinomycin: The first bifunctional intercalating agent in clinical trials

SummaryEchinomycin is a quinoxaline antibiotic that was originally isolated from Streptomyces echinatus. Based on its antitumor activity against two i.p. implanted murine tumors, the B16 melanoma, and the P388 leukemia, it was brought into clinical trials by the National Cancer Institute. Recent studies on its cytotoxic action have related its antitumor activity with its ability to bifunctionally intercalate with double stranded DNA.Toxicologic studies were carried out in CDF1 mice and beagle dogs using intravenous injections. For the mice studies the dose ranges were 288–692 mcg/kg (864–2076 mcg/m2) by single bolus, and 112–254 mcg/kg/day (336–762 mcg/m2/day) for five consecutive days. In the dog, dose ranges studied were 8.9–89.4 mcg/kg (178–1788 mcg/m2) by single bolus, and 3.4–33.5 mcg/kg/day (68–670 mcg/m2/day) for five consecutive days. The major toxic effects were found in the gastrointestinal, hepatic, and lymphoreticular systems. These were reversible at all but the highest dose, in dogs that had been treated for five consecutive days.Phase I clinical trials using various intravenous schedules were sponsored by the National Cancer Institute. Nausea, vomiting, reversible liver enzyme abnormalities, and allergic reactions were the most common toxicities encountered. Based on results from these studies, the National Cancer Institute has recently begun phase II trials in a broad range of diseases. These trials will further characterize echinomycins toxic effects and its antitumor activity.

Flavone acetic acid (LM 975, NSC 347512) A novel antitumor agent

SummaryFlavone acetic acid (FAA) is a synthetic flavonoid compound which has recently begun clinical trials as an antitumor agent based on its striking activity in solid tumor model systems. The pharmacologic behavior of FAA in animals appears to be predictive of both its cytotoxic efficacy and its toxicity to normal tissues (principally the central nervous system and gastrointestinal tract). The design and conduct of phase I studies in man are based upon these principles, with the goal of maximizing their safety and efficacy.

Trimetrexate: a new antifol entering clinical trials

SummaryTrimetrexate, a 2,4-diaminoquinazoline derivative, is a new antifol recently introduced into clinical trials. It differs from methotrexate principally in its transport (not carrier-mediated), and its intracellular retention (not polyglutamylated). Trimetrexate is active against tumors which are methotrexate-resistant on the basis of impaired transport, and has a broader range of antitumor activity in preclinical models. Animal studies predict toxicity principally to the central nervous system, gastrointestinal tract and bone marrow.

Tiazofurin: A new antitumor agent

SummaryTiazofurin is an interesting drug now entering Phase I trials, with marked preclinical antitumor activity against P388 and L1210 leukemias, and the Lewis lung carcinoma. Schedule dependency favoring frequent administration has been noted.The drug has a novel mechanism of action, being metabolized to an inhibitory cofactor of inosine monophosphate dehydrogenase.Tiazofurin is widely distributed after i.v. administration exhibiting a triphasic pattern of plasma decay, with a terminal half-life of 3–16 h in the three species studied. Approximately 90% of the drug was excreted unchanged in the urine within 24 h. A significant potential for the slower release of intracellularly retained drug exists.Anticipated organ toxicities based on the studies described include myelotoxicity, hepatotoxicity and nephrotoxicity. These were mild and reversible at lower doses, and were not seen at levels corresponding to the starting doses in man. A potential for hyperuricemia exists; this should be easily controllable by the use of allopurinol, without compromising the drugs antitumor effect.Phase I trials under the sponsorship of the NCI are underway in a number of institutions.

Arabinosyl-5-azacytosine: A novel nucleoside entering clinical trials

Arabinosyl-5-azacytosine is a new compound which has been selected by the Division of Cancer Treatment, National Cancer Institute for clinical development as an antineoplastic agent based on its high degree of activity against a broad range of tumor types in preclinical studies. Therapeutic activity has been observed against murine and human leukemias, transplantable murine solid tumors, and human tumor xenografts. Arabinosyl-5-azacytosine exhibited a broader spectrum of activity against human solid tumors than cytosine arabinoside. Arabinosyl-5-azacytosine is phosphorylated to the nucleotide level by deoxycytidine kinase. Upon further anabolism to the triphosphate level, it can be incorporated into DNA. The mechanism of cytotoxicity is thought to be related to inhibition of DNA synthesis. Leukemic and solid tumor cell lines that are resistant to cytosine arabinoside due to deletion of deoxycytidine kinase activity are cross-resistant to arabinosyl-5-azacytosine. Unlike cytosine arabinoside, arabinosyl-5-azacytosine does not readily undergo deamination. Schedule dependence has been demonstrated in mice bearing L1210 leukemia, with superior activity seen with multiple doses administered on each treatment day compared to administration of larger but less frequently administered doses. From preliminary data in solid tumor models, however, antitumor activity did not appear to be superior with continuous infusion compared to that observed on a bolus schedule. Preclinial toxicology studies indicated that the bone marrow and gastrointestinal tract were the main target organs. A single large dose of arabinosyl-5-azacytosine could be tolerated by both mice and dogs. When administered as a continuous infusion, the toxicity was related to both the dose and duration of exposure, suggesting that toxicity resulted from a critical time above a threshold concentration as opposed to the total area under the concentration-time curve. Phase I clinical trials have been initiated to determine the maximum tolerated dose on a low dose continuous infusion schedule for 72 hours and also on a high dose short infusion daily times five schedule.

Clinical toxicity associated with tiazofurin

SummaryTiazofurin, an investigational antimetabolite, is undergoing clinical evaluation in leukemia. We analyzed the data base of 198 patients entered in Phase I trials to characterize the incidence and severity of toxicities associated with tiazofurin according to dose and schedule. Severe myelosuppression occurred infrequently, and was not dose-dependent. A five day bolus schedule had a higher incidence of severe or life-threatening neutropenia than other schedules. Tiazofurin produced lymphopenia which was not dose-dependent in the range of 23–36% decrease from baseline, and the effect on lymphocyte count was generally greater than the decline in neutrophil count. Non-hematologic toxicity of a moderate or worse severity (≥ grade 2) included nausea and vomiting (18% of all courses), serum transaminase elevations (SGOT, 16%; SGPT, 9%), rash (9%), stomatitis (3%), conjunctivitis (3%), headache (10%), other signs of central nervous system toxicity (8%), and cardiac toxicity, primarily pleuropericarditis (4%). Dose-related cutaneous toxicity, headache, and nausea and vomiting were evident in the five day bolus schedule, and myalgia was more frequently reported at higher doses on the single dose schedule. The five day continuous infusion (CI) schedule had a higher incidence of neurotoxicity, cardiac toxicity, SGPT elevations and ocular toxicity than the daily for five days bolus schedule, but none of these differences attained statistical significance. Although the peak plasma concentrations of tiazofurin achieved with the five day bolus schedule were 3-fold higher than the steady-state plasma levels seen with an equal dose given by CI, the area under the concentration-time curve (AUC) was approximately 1.6-fold higher with CI. These observations suggest that both high peak plasma concentrations (above 400 uM) and prolonged exposure to plasma levels exceeding 50 uM may result in a higher incidence of serious non-hematologic toxicity.

Induction and development of mouse liver glutathione S-transferase activity

Mouse liver glutathione S-transferase activity at birth was 1/10 that of adults, and increased steadily with each successive week of age until adult values were reached at 8 weeks. Activity was inducible with phenobarbital; however, the percentage increase in activity was dependent upon substrate. 2 distinct peaks of transferase activity were obtained on CM-cellulose chromatography. The ratios of transferase activity observed for each peak demonstrated that glutathione S-transferase activity in mouse liver is associated with at least 2 distinct proteins with differing substrate specificities.

Spiromustine: a new agent entering clinical trials

SummarySpiromustine is a new alkylating agent, of interest since it was rationally designed as a lipophilic compound capable of penetrating the CNS. This lipophilicity may also enhance alkylating activity against tumors other than brain tumors.Preclinical screening has shown activity against a variety of tumors, including an intracranially implanted ependymoblastoma. Alkylating activity has been demonstrated in an intracerebral glioma in the rat. Spiromustine is a cell cycle non-specific agent. Animal pharmacology studies have shown a biphasic plasma decay curve, with hepatic metabolism and excretion, an enterohepatic circulation of metabolites, and approximately 50% renal excretion of unchanged drug. Toxicology studies in mice, rats and dogs showed that dose-related myelosuppression, and neurotoxicity predominated; other organ toxicities were mild.Spiromustine is currently entering Phase I clinical trials on a variety of schedules.

Absorption of m-AMSA and its biliary metabolites from the rat small intestine

The ability of orally administered m-AMSA (NSC-249992) to be absorbed was examined in the rat. Studies using isolated in situ intestinal loops showed that m-AMSA (25 μM) was absorbed equally well in jejunum and ileum and that absorption was 50% complete in 5–10 min and 90% complete by 60 min. In the intact rat, orally administered m-AMSA was found to be 93% absorbed at a dose of 50 mg/kg and 82% absorbed at a dose of 200 mg/kg. Absorption was measured 8 hr after dosing. m-AMSA biliary metabolites were found to be poorly absorbed by the gut. Following a dose of m-AMSA (10 mg/kg, i.v.), rats were observed for 4 hr and less than 10% of the available m-AMSA biliary metabolite was absorbed. Enterohepatic circulation of m-AMSA appears to be insignificant.

Studies on the disposition of 2,3-dihydro-1H-imidazo [1,2-b] pyrazole in rodents

Abstract 2,3-Dihydro-1H-imidazo [1,2-b] pyrazole (IMPY, NSC-51143) is a new ribonucleotide reductase inhibitor, presently undergoing clinical evaluation, that exhibits prolonged in vivo antitumor activity in experimental animals. Disposition studies were initiated to determine if the prolonged in vivo antitumor activity of IMPY could be explained by its pharmacologic properties. Plasma disappearance curves of radioactivity were biphasic after the i.v. administration of 100, 250 or 500 mg/kg to rats and 250 mg/kg to mice. The distribution phase was rapid in each case ( t 1 2 of approximately 30 min or less), followed by a prolonged elimination phase. Radioactivity was distributed to all tissues of rats and mice including brain after an i.v. dose of 250 mg/kg. In rats, 67.8% of the administered radioactivity was excreted in the urine during the first 24 hr. By 120 hr 74.3% had been excreted via the urine compared with 11.1% in the fees, the latter by biliary excretion. The chromatographic profile of the urine collected from rats and mice 4 hr after drug administration indicated extensive metabolism. Thus, the prolonged plasma and tissue levels of parent IMPY and its metabolites can account for the prolonged duration of cytotoxic activity in vivo.