Thiopurine metabolism and identification of the thiopurine metabolites transported by MRP4 and MRP5 overexpressed in human embryonic kidney cells.

Mercaptopurines have been used as anticancer agents for more than 40 years, and most acute lymphoblastic leukemias are treated with 6-mercaptopurine (6MP) or 6-thioguanine (TG). Overexpression of the two related multidrug resistance proteins MRP4 and MRP5 has been shown to confer some resistance against mercaptopurines, which has been attributed to extrusion of mercaptopurine metabolites by these transporters. We have analyzed the mercaptopurine metabolites formed in human embryonic kidney cells and determined which metabolites are extruded by MRP4 and MRP5. Incubation with 6MP led to the formation of thioinosine and thioxanthosine metabolites and we found that thio-IMP was transported by both MRP4 and MRP5; MRP5 showed the highest transport rate. In contrast, only MRP5 transported thioxanthosine monophosphate (tXMP). During incubation with TG, the monophosphorylated form of thioguanosine was transported by both MRP4 and MRP5; the highest transport rate was for MRP4. Similarly, only 6-methyl-thio-IMP was formed during incubation with 6-methyl mercaptopurine riboside. This compound was a substrate for both MRP4 and MRP5; MRP4 showed the highest transport rate. Our results show that all major thiopurine monophosphates important in the efficacy of mercaptopurine treatment are transported by MRP4 and MRP5, although the substrate specificity of the two transporters differs in detail.

Chemical stability of two sterile, parenteral formulations of cyclophosphamide (Endoxan) after reconstitution and dilution in commonly used infusion fluids.

The commercially available parenteral dosage forms of cyclophosphamide (Endoxan, Cycloblastine) are manufactured by an aseptic dry-filling technique and exhibit a slow dissolution rate. A novel dosage form has been developed by one of the manufacturers based on the technique of freeze drying. Dissolution rates of both types of formulations were determined and it was shown that the freeze-dried formulation dissolves more rapidly, within 20 seconds, while it takes at least three minutes to dissolve the dry-filled formulation. The chemical stabilities of the cyclophosphamide solutions, obtained after reconstitution and/or dilution of both formulations, have been investigated and tested as a function of drug concentration (20 and 1 mg/mL), solvent (water, 0.9% sodium chloride, 5% dextrose), container material (glass and polyvinyl chloride (PVC)), light conditions (normal room fluorescent light/dark) and temperature (4 degrees, 20-22 degrees and 37 degrees C). The test solutions were analyzed by a stability-indicating reverse phase high performance liquid chromatographic method with ultraviolet detection at 214 nm. Cyclophosphamide solutions (solvent: water; drug concentration; 20 mg/mL) are stable when stored for seven days at 4 degrees C in the dark. At higher temperatures degradation occurred during the test period with 10% loss after seven days at ambient temperature and 50% loss after seven days storage at 37 degrees C. Similar data were found in admixtures with 5% dextrose and 0.9% sodium chloride and initial drug concentration of 1 mg/mL. There are no significant differences in chemical stability between the solutions obtained from reconstitution and dilution of the dry-filled and lyophilized formulations.(ABSTRACT TRUNCATED AT 250 WORDS)