Lipid hydroperoxides greatly increase the rate of oxidative catabolism of all-trans-retinoic acid by human cell culture microsomes genetically enriched in specified cytochrome P-450 isoforms.

Cytochrome P-450 enzymes have been implicated in the oxidative catabolism of all-trans-retinoic acid (RA), a process that is accelerated by exposure to RA in cultured cells and rodents, and also in patients receiving RA as treatment for cancer (J.F.R. Muindi et al., Cancer Res., 52: 2138, 1992; Blood, 79: 299, 1992). Accelerated oxidation of RA could arise from an induction of RA-catabolizing P-450 isoforms or from an increase in oxidative cofactors. We have examined the efficiency of NADPH/O2 and lipid hydroperoxides (LOOH) to support oxidation of RA using human cell microsomes genetically enriched in different P-450 isoforms. The observed rate of RA oxidation using the NADPH/O2 system was slow for all isoforms (6-23 pmol/mg protein/min). LOOH-mediated oxidation was much faster (24-1078 pmol/mg protein/min), not isoform specific, but dependent upon the chemical nature of the LOOH. The order of efficiency of RA oxidation using LOOH was 13-hydroperoxy[S-(E,Z)]-9,11-octadecadienoic acid > 5-hydroperoxy[S-(E,Z,Z,Z)]-6,6,11,14-eicosatetraenoic acid > prostaglandin G2 > cumene hydroperoxide > tert-butylhydroperoxide > H2O2. Whereas submicromolar concentrations of 13-hydroperoxy[S-(E,Z)]- 9,11-octadecadienoic and 5-hydroperoxy[S-(E,Z,Z,Z)]-6,6,11,14- eicosatetraenoic acid oxidized RA at appreciable rates, micromolar concentrations were required for the other LOOH. These observations suggest that physiological LOOH, generated by the arachidonic acid-lipoxygenase system, may be involved in the self-induced oxidative catabolism of RA.

Elevated plasma lipid peroxide content correlates with rapid plasma clearance of all-trans-retinoic acid in patients with advanced cancer.

The addition of lipid hydroperoxides greatly accelerates the rate of oxidative catabolism of all-trans-retinoic acid (RA) in human cell microsomes; hydroperoxy metabolites of the arachidonate cascade are particularly active in the microsomal system. We have measured the plasma content of lipid peroxides in cancer patients during the course of therapy with RA, seeking to assess whether a correlation exists between the rate of oxidative catabolism of exogenously administered RA and whole body lipid peroxide levels. The assay used for plasma lipid peroxides is the capacity to react with thiobarbituric acid under specified conditions; the result is expressed as TBARS (thiobarbituric acid reactive substances). RA administration produced its own accelerated clearance RA within 72 h. Patients were considered to have "normal" or "rapid" baseline catabolism of RA if their Day 1 area under RA concentration over time curve was greater or less than 300 ng.h/ml, respectively. The mean plasma TBARS levels were: 12 normal volunteers = 0.14 microM; 19 "normal" RA catabolizers = 0.25 microM; and 14 "rapid" catabolizers = 0.82 microM. P = 0.008 (rapid catabolizers versus normal volunteers) and 0.05 (rapid catabolizers versus normal catabolizers). Repeat TBARS determinations were made during the course of therapy in 17 patients, all of whom converted to "rapid" RA catabolism on therapy. An increase in plasma TBARS levels > or = 20% of baseline was observed in 5 of 5 prostate cancer patients and 8 of 12 lung cancer patients treated with continuous RA therapy for 2 and 4 weeks, respectively. These observations support the hypothesis that high levels of lipid peroxides and rapid oxidative catabolism of RA are positively correlated.

Clinical pharmacology of deoxyspergualin in patients with advanced cancer.

Pharmacokinetic studies were carried out in 25 patients with advanced cancer receiving deoxyspergualin (DSG), a candidate anticancer agent, in a dose-finding Phase I study. The dosage range explored was 80 to 2160 mg/m2/day for 5 days by continuous i.v. infusion. The drug levels in plasma and urine were measured by high-performance liquid chromatography with postcolumn derivatization and fluorescence detection. One drug metabolite was demonstrated in plasma and urine of treated patients. This metabolite was extracted from urine and purified to homogeneity; thereafter, it was examined by high-performance liquid chromatography, nuclear magnetic resonance, and fragmentation mass spectrometry and was demonstrated to be identical to chemically synthesized desaminopropyl-DSG. The mean steady state plasma concentrations of DSG ranged from 0.28 to 11.1 microM at, respectively, the 80- and 2160-mg/m2 dosage levels. The plasma concentration at steady state and the area under the plasma concentration versus time curve of DSG were proportional to dose (r = 0.97). Following discontinuance of the infusion, DSG was cleared from the plasma in a biexponential fashion. The mean total body clearance was 364 +/- 78 ml/min/m2. Desaminopropyl-DSG was formed extensively at all dosage levels; mean steady state plasma levels of this metabolite reached a plateau 2.65 microM at a dose of 720 mg/m2/day and did not rise with further dose increments. The urinary content of DSG was examined in 20 patients over the dosage range from 160 to 960 mg/m2/day; in this group less than 10% of the administered dose was excreted as DSG. In four patients at the 720- and 960-mg/m2/day dosage levels, the total DSG plus metabolite excretion ranged from 7 to 18% of the administered dose, with comparable quantities occurring as the parent drug and desaminopropyl-DSG.

In vitro differential metabolism of merbarone by xanthine oxidase and microsomal flavoenzymes. The role of reactive oxygen species.

Merbarone (MB), a nonsedating derivative of thiobarbituric acid, was recently found to induce profound hypouricemia. When incubated with xanthine oxidase (XO) and hypoxanthine in vitro, MB is both an inhibitor of XO and degraded by the XO-hypoxanthine interaction. Compared with allopurinol (Ki = 0.025 microM), MB is a very weak inhibitor of XO (Ki = 51 +/- 8 microM). MB interacts with XO in the presence of hypoxanthine to yield three chromatographically separate products. One of these products has been identified by HPLC retention time and spectral characteristics as 2-oxo-2-desthiomerbarone (2-oxo-MB). The other two products are thought to be S-oxide intermediates in the oxidative desulfuration of this drug. Formation of these products was blocked by catalase, suggesting that the conversion was dependent on reactive oxygen species (especially H2O2) generated by the hypoxanthine-XO system. This suggestion was confirmed by incubating MB with H2O2. In vitro studies with rat liver microsomes have documented the formation of 2-oxo-MB and 4'-OH-MB (4'-OH-MB), the latter being identified by the characteristic HPLC retention time of its acetylated derivative. The formation of 4'-OH-MB has many characteristics of a cytochrome P-450-dependent monooxygenase reaction (NADPH requirement and SKF 525-A inhibition); formation of 2-oxo-MB occurs by a different mechanism that is, as yet, uncharacterized. Incubation of kidney microsomes with MB generated 2-oxo-desthiomerbarone but no detectable 4'-OH-MB.(ABSTRACT TRUNCATED AT 250 WORDS)

Methotrexate with thymidine, inosine, and allopurinol rescue: a phase I clinical study.

A phase I clinical study of a combination of thymidine (TdR), inosine (IR), and allopurinol used as rescue from 24-hour infusions of methotrexate (MTX) was undertaken following animal studies that had shown a better preservation of antitumor activity with this system than with folinic acid. TdR and IR were given in the dose ratio 5:1 by weight throughout. Rescue from MTX, 400 mg/m2, could be achieved with a TdR dose in the combination of 1 g/m2/24 hours. This same rescue was also effective when the MTX dose was increased to 800 mg/m2, but only partly effective at an MTX dose of 1.5 g/m2. It was not necessary to raise the levels of circulating nucleosides significantly above normal to achieve rescue. MTX-related skin lesions occurred more frequently than might be expected with the MTX dose used.