Validation and use of three complementary analytical methods (LC-FLS, LC-MS/MS and ICP-MS) to evaluate the pharmacokinetics, biodistribution and stability of motexafin gadolinium in plasma and tissues.

Liquid chromatography-fluorescence (LC-FLS), liquid chromatography-tandem mass spectrometry (LC-MS/MS) and inductively coupled plasma-mass spectrometry (ICP-MS) methods were developed and validated for the evaluation of motexafin gadolinium (MGd, Xcytrin) pharmacokinetics and biodistribution in plasma and tissues. The LC-FLS method exhibited the greatest sensitivity (0.0057 microg mL(-1)), and was used for pharmacokinetic, biodistribution, and protein binding studies with small sample sizes or low MGd concentrations. The LC-MS/MS method, which exhibited a short run time and excellent selectivity, was used for routine clinical plasma sample analysis. The ICP-MS method, which measured total Gd, was used in conjunction with LC methods to assess MGd stability in plasma. All three methods were validated using human plasma. The LC-FLS method was also validated using plasma, liver and kidneys from mice and rats. All three methods were shown to be accurate, precise and robust for each matrix validated. For three mice, the mean (standard deviation) concentration of MGd in plasma/tissues taken 5 hr after dosing with 23 mg kg(-1) MGd was determined by LC-FLS as follows: plasma (0.025+/-0.002 microg mL(-1)), liver (2.89+/-0.45 microg g(-1)), and kidney (6.09+/-1.05 microg g(-1)). Plasma samples from a subset of patients with brain metastases from extracranial tumors were analyzed using both LC-MS/MS and ICP-MS methods. For a representative patient, > or = 90% of the total Gd in plasma was accounted for as MGd over the first hour post dosing. By 24 hr post dosing, 63% of total Gd was accounted for as MGd, indicating some metabolism of MGd.

Population pharmacokinetics of motexafin gadolinium in adults with brain metastases or glioblastoma multiforme.

The purpose of this study was to determine clinical variables affecting motexafin gadolinium (MGd) pharmacokinetics. Motexafin gadolinium (4-5.3 mg/kg/d) was administered intravenously for 2 to 6.5 weeks. Plasma samples from 3 clinical trials were analyzed for MGd using liquid chromatography/mass spectroscopy. The pooled data were analyzed using population pharmacokinetic (POP-PK) methods. The POP-PK model included 243 patients (1575 samples). Clearance (CL) was 14% lower in women, but weight-normalized clearance was only 5% lower in women. Clearance decreased with increasing alkaline phosphatase, increasing age, and decreasing hemoglobin. Administration of phenytoin increased CL by approximately 30%. Central compartment volume (V1) was 21% lower in women and increased with increasing serum creatinine. For all covariates, except sex and phenytoin, the predicted change in CL or V1 (5th and 95th percentiles) varied < or =13% from the population mean CL or V1 estimate. It was concluded that a 3-compartment, open, POP-PK model predicts small but significant effects of age, sex, alkaline phosphatase, hemoglobin, serum creatinine, and phenytoin on MGd pharmacokinetics.

Reductase-mediated metabolism of motexafin gadolinium (Xcytrin) in rat and human liver subcellular fractions and purified enzyme preparations.

The biotransformation of motexafin gadolinium (MGd, Xcytrin) was investigated in subcellular rat and human liver fractions. Microsomal MGd metabolism was dependent on NADPH in both species. Cytosolic metabolism in rat and human livers was dependent on NADPH or NADH. Under anaerobic conditions, MGd metabolism increased in liver microsomes and purified enzyme preparations. Cytochrome P450 (CYP450) inhibitors ketoconazole, proadifen, and carbon monoxide increased NADPH-dependent MGd metabolism in microsomes. Treatment of rats with beta-naphthoflavone increased cytosolic metabolism of MGd twofold, but had no effect on microsomal metabolism. Conversely, in liver preparations from phenobarbital treated rats microsomal metabolism of MGd was enhanced twofold, but not in cytosolic preparations. Purified CYP450 reductase from phenobarbital-treated rabbit or untreated human livers metabolized MGd suggesting involvement of CYP450 reductase. Dicumarol, a potent DT-diaphorase inhibitor, inhibited MGd metabolism in both rat and human liver cytosol. These data suggest MGd metabolism in rat liver involves CYP450 reductase and/or DT-diaphorase. In human liver preparations only CYP450 reductase is directly involved in MGd metabolism. A metabolite identified in microsomes and cytosol is a metal-free, reduced form of MGd, indicating that both enzymes generate metabolite 1, which appears to be PCI-0108, a synthetic precursor to MGd.