HPLC Determination of 6-Thiouric Acid and 6-Mercaptopurine in Organ Transplant Patient Serum

Optimization and validation of an RP-HPLC method for the estimation of 6-mercaptopurine in bulk and pharmaceutical formulations

A reverse phase HPLC method is described for the determination of 6-mercaptopurine in bulk and tablets. Chromatography was carried on a C18 column using a mixture of acetonitrile and 0.05 mol/L sodium acetate buffer (10:90 v/v) as the mobile phase at a flow rate of 1 mL/min-1 with detection at 324 nm. The retention time of the drug was 3.25 min. The detector response was linear in the concentration of 0.01-5 μg/mL. The limit of detection and limit of quantification were 17 and 52 ng/mL respectively. The method was validated by determining its sensitivity, linearity, accuracy and precision. The proposed method is simple, economical, fast, accurate and precise and hence can be applied for routine quality control of mercaptopurine in bulk and tablets.

Therapeutic drug monitoring of antimetabolic cytotoxic drugs

Therapeutic drug monitoring is not routinely used for cytotoxic agents. There are several reasons, but one major drawback is the lack of established therapeutic concentration ranges. Combination chemotherapy makes the establishment of therapeutic ranges for individual drugs difficult, the concentration-effect relationship for a single drug may not be the same as that when the drug is used in a drug combination. Pharmacokinetic optimization protocols for many classes of cytotoxic compounds exist in specialized centres, and some of these protocols are now part of large multicentre trials. Nonetheless, methotrexate is the only agent which is routinely monitored in most treatment centres. An additional factor, especially in antimetabolite therapy, is the existence of pharmacogenetic enzymes which play a major role in drug metabolism. Monitoring of therapy could include assay of phenotypic enzyme activities or genotype in addition to, or instead of, the more traditional measurement of parent drug or drug metabolites. The cytotoxic activities of mercaptopurine and fluorouracil are regulated by thiopurine methyltransferase (TPMT) and dihydropyrimidine dehydrogenase (DPD), respectively. Lack of TPMT functional activity produces life-threatening mercaptopurine myelotoxicity. Very low DPD activity reduces fluorouracil breakdown producing severe cytotoxicity. These pharmacogenetic enzymes can influence the bioavailability, pharmacokinetics, toxicity and efficacy of their substrate drugs.

Plasma concentrations and organ distribution of thiopurines after oral application of azathioprine in mice

The plasma concentrations and tissue distribution of thiopurines were studied in mice after oral administration of 50 mg/kg azathioprine (AZA) using HPLC analysis. Peak concentrations of AZA and three other thiopurine metabolites in plasma [thiouric acid (TUA) greater than 6-mercaptopurine (6-MP) greater than AZA greater than 8-hydroxy-AZA] were observed as early as 10 min after drug application, thus indicating fast absorption and extensive metabolism of AZA, and were followed by a rapid decline. The extraction of thiopurines from organs (intestinal mucosa, liver, kidney, testes, spleen, and bone marrow) and from red blood cells (RBCs) was preceded by an acid hydrolysis procedure resulting in the release of thiopurine bases from their corresponding ribonucleotides. 6-MP, 6-thioxanthene (6-TX), 6-thioguanine (6-TG), TUA, and 8-hydroxy-6-MP (8-OH-6-MP) were extracted from the organs, whereas only 6-MP and 8-OH-6-MP were found in the processed RBCs. Initially, high concentrations of TUA, the endpoint of metabolic AZA degradation, were detected in the intestinal mucosa and in the liver. This provides evidence for a first-pass metabolism of AZA in these two organs. The initial concentrations of 6-MP extracted from the organs were about 10-fold those found in plasma. This indicates rapid cellular uptake of 6-MP and an accumulation of 6-MP derivatives that can be explained by formation of the 6-MP ribonucleotide thioinosine monophosphate (TIMP). With the exception of plasma and RBCs, 6-TG, which may originate from intracellular 6-thioguanosine nucleotides (TGNs), was extracted from all organs examined in the study. From the sequence of appearance of 6-MP, 6-TX, and 6-TG extracted from spleen and bone marrow homogenates, it can be assumed that formation of TGN occurs via the nucleotide interconversion pathway TIMP----6-thioxanthosine monophosphate----6-thioguanosine monophosphate. The highest concentrations of 6-TG derivatives were found in the spleen and bone marrow. This correlates with the clinical and experimental observation that AZA cytotoxicity mainly affects bone-marrow stem cells and lymphocytes and supports the hypothesis (derived from in vitro experiments) that the incorporation of TGN into DNA is the cytotoxic mechanism of AZA and 6-MP.

Azathioprine metabolism: pharmacokinetics of 6-mercaptopurine, 6-thiouric acid and 6-thioguanine nucleotides in renal transplant patients

Despite extensive clinical experience with azathioprine (AZA), the disposition of various AZA metabolites remains obscure. We therefore evaluated the pharmacokinetics of three AZA metabolites: 6-mercaptopurine (6-MP), the immediate metabolite; 6-thiouric acid (6-TU), the final end product; and 6-thioguanine nucleotides (TGN), the active moiety; in eight renal transplant patients after oral administration of AZA. The low peak plasma 6-MP level of 73.7 +/- 23.7 ng/mL (mean +/- SD) and the short half-life (t1/2) of 1.9 +/- 0.6 hours suggest rapid conversion of 6-MP to other metabolites. A peak plasma 6-TU concentration of 1210 +/- 785 ng/mL was observed at 3.5 +/- 0.6 hours after the AZA dose. The strong correlation between 6-TU t1/2 and serum creatinine (r = 0.98, P = .0008) supported our previous work showing that 6-TU is primarily excreted by the kidneys. The total TGN levels in red blood cells (RBCs) in each patient remained largely unchanged over 24 hours with the intraindividual coefficient of variation ranging from 4.4% to 29.8%. In comparison, the mean TGN level varied considerably between patients, and ranged from undetectable to 413 pmol per 8 X 10(8) RBCs. However, there was no apparent correlation between white cell counts on day 0 (P greater than .5), day 7 (P greater than .5), or day 14 (P greater than .5) and RBC TGN level. The persistence of TGN in body tissues thus provides a pharmacokinetic rationale for the conventional once or twice daily AZA regimen.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of 6-thiouric acid and 6-mercaptopurine in renal allograft recipients after oral administration of azathioprine

The immunosuppressive activity of azathioprine (AZA) is unpredictable and depends on the formation of intracellular thiopurine ribonucleotides. However, the quantification of these active thiopurines presents difficult analytical problems. It has recently been postulated that plasma concentrations of 6-thiouric acid (6-TU) and 6-mercapto-purine (6-MP), metabolites of AZA, may provide more readily measurable indices of the pharmacologic activity of AZA. In order to evaluate the utility of 6-TU and 6-MP plasma concentrations in monitoring AZA therapy, we studied their pharmacokinetics in 6 renal transplant patients, and their in vitro immunosuppressive potency in a mixed lymphocyte proliferation assay. A peak plasma 6-TU concentration of 710.7 ng/ml was observed at 3.8 h after oral dosing. Good correlation was observed between the elimination t1/2 of 6-TU and serum creatinine, and between AUC over 24 h and serum creatinine. However, we did not observe a second peak in plasma 6-TU concentration that could be attributed to the degradation of active AZA metabolites. 6-MP plasma concentrations in the patients were low (mean peak concentration 36.0 ng/ml) and rapidly disappeared within 8 h. In vitro immunosuppressive activity could not be demonstrated for 6-TU over a concentration range of 1.25 ng/ml to 0.25 mg/ml. We conclude that 6-TU is pharmacologically inert and is primarily eliminated by the kidneys. Our findings currently do not support the use of plasma concentrations of 6-TU or 6-MP to monitor AZA therapy. In order to optimize AZA therapy, analytical techniques that are technically feasible and that can directly quantify the active intracellular thiopurines are being explored.