Toxicity of a quinocarmycin analog, DX-52-1, in rats and dogs in relation to clinical outcome.

PURPOSE: Quinocarmycin analog DX-52-1 is a cyanated derivative of quinocarmycin, a compound isolated from cultures of Streptomyces melanovinaceus. DX-52-1 was selected for preclinical development because it showed efficacy against melanoma cell lines in the NCI human tumor cell screen and melanoma xenografts in mice. This report describes studies in rats and dogs to determine the maximum tolerated dose (MTD) and identify dose-limiting toxicities (DLT) in each species in different regimens to establish a safe starting dose and potential target organs of DX-52-1 for phase I clinical trials. METHODS: DX-52-1 was administered to Fischer 344 rats using repeated intravenous (i.v.) slow bolus injections following q3hx3 and q3hx3,q7dx3 regimens, and to beagle dogs using a single injection, 6-h continuous i.v. infusion (c.i.v.) and weekly 6-h c.i.v. for 3 weeks. Endpoints evaluated included clinical observations, body weights, hematology, serum clinical chemistry, and microscopic pathology of tissues. RESULTS: The MTD of DX-52-1 was a total dose of 18 mg/m(2) body surface area for q3hx3 administration in rats and 30 mg/m(2) for a single c.i.v. administration in dogs. The total dose MTD for rats on a weekly (q3hx3,q7dx3) regimen was 54 mg/m(2), and for dogs on the weekly x3 (6-h c.i.v.) infusion was 60 mg/m(2). In rats, significant elevations in blood urea nitrogen and creatinine were observed together with acute renal tubular necrosis histologically. Modest increases in liver enzymes were also observed, as were decreases in reticulocytes that were unaccompanied by histologic changes in liver and bone marrow. In dogs, adverse signs included vomiting/retching, diarrhea, and transient hypothermia; also red blood cells, hemoglobin, hematocrit, and lymphocytes were decreased. Histologic evaluation of tissues from dogs revealed necrosis and cellular depletion of the bone marrow, and extensive damage to the entire gastrointestinal tract, including marked cellular necrosis of the mucosa and lymphoid necrosis of the gastrointestinal associated lymphoid tissue. Destruction of the mucosal lining of the intestinal tract was likely responsible for dehydration, toxemia, septicemia, and shock seen in moribund dogs. CONCLUSIONS: The MTD values were comparable between rats and dogs given roughly similar dose regimens (single dose or weekly) and both species tolerated a higher total dose with weekly administration. However, the principal target organ responsible for DLT in rats was the kidney, whereas in dogs, the most severe effects were on the gastrointestinal tract and bone marrow. Both renal and gastrointestinal toxicities were reported in patients after 6-h c.i.v. infusions in a limited phase I clinical trial, indicating that neither animal model alone was predictive of DX-52-1-induced toxicity in humans, and that both species were required to define human toxicity.

Synergistic bone marrow toxicity of pyrimethamine and zidovudine in murine in vivo and in vitro models: mechanism of toxicity.

Pyrimethamine (Pyr) is commonly used for treatment of toxoplasmic encephalitis in AIDS patients; however, in two clinical studies, an increased number of deaths were observed when Pyr was coadministered with zidovudine (ZDV). The BALB/c mouse was chosen as a model to study the mechanism underlying the unexpected toxicity from coadministration of these drugs. Daily administration by oral gavage of 60 mg/kg Pyr and 240 mg/kg ZDV resulted in 100% lethality after 30 days. These dose levels produced no effect when the drugs were given individually for the same period. Administration of combinations of Pyr and ZDV resulted in macrocytic anemia and leukopenia with synergistic decreases in lymphocyte and neutrophil numbers. To examine the mechanism of this hematotoxicity at the cellular level, mouse bone marrow colony-forming unit (mCFU) assays were employed. A combination of ZDV with various concentrations of Pyr resulted in synergistic decreases in numbers of erythroid and granulocyte-macrophage precursors (mCFU-E and mCFU-GM). mCFU-GM precursors appeared more sensitive than erythroid precursors to combinations of Pyr and ZDV. Incorporation of (14)C-ZDV into cellular DNA was increased in a dose-dependent manner in the presence of increasing concentrations of Pyr in the mCFU-GM assay. This suggested that inhibition of dihydrofolate reductase by Pyr and accompanying inhibition of dTTP synthesis allows preferential incorporation of ZDV into DNA, with resulting strand breakage and cell death. (14)C-ZDV incorporation was also observed when human GM cultures were analyzed, however, incorporation was less and required higher concentrations of Pyr.

The potential for an interaction between MRP2 (ABCC2) and various therapeutic agents: probenecid as a candidate inhibitor of the biliary excretion of irinotecan metabolites.

Irinotecan hydrochloride (CPT-11) is an anticancer agent with unpredictable bouts of diarrhea as a dose-limiting toxic side-effect. Since the biliary excretion of its active metabolite (SN-38) and SN-38 glucuronide (SN38-Glu), which are mediated by the multidrug resistance associated protein-2 (MRP2/ABCC2), has been proposed to be related to this gastrointestinal toxicity, we have attempted here to examine the potential of various therapeutic agents to interact with the biliary excretion in order to identify MRP2 inhibitors to prevent this toxicity. The inhibition constants (K(i)) of 26 compounds were examined for the transport of a typical MRP2 substrate in isolated canalicular membrane vesicles. Of these, 13 compounds inhibited the transport with K(i) values from 0.0461 to 281 microM. Three inhibitors (probenecid, sulfobromophthalein and glycyrrhizin) were also found to inhibit the biliary excretion of SN-38 and SN38-Glu in rats in vivo, and the degrees of inhibition were compatible with the estimated values based on the ratios of K(i) and unbound concentrations in circulating plasma. A similar estimation of the potential inhibitory effect in human was also examined by considering both the K(i) of each therapeutic agent and its unbound concentration both in circulating plasma and the inlet to the liver. The predicted degrees of inhibition by most compounds were minimal whereas approximately 75% inhibition was predicted for probenecid. Thus, probenecid may be a candidate which can be used clinically to inhibit the biliary excretion of CPT-11 metabolites, whereas an interaction between most of the other compounds and MRP2 is more unlikely.