Pharmacodynamic monitoring of immunosuppressive drugs

Pharmacodynamic (PD) monitoring measures the biological response to a drug, which alone--or coupled with pharmacokinetics--provides a novel method for optimization of drug dosing. PD monitoring has been investigated by us and other investigators primarily for four immunosuppressive drugs: cyclosporine (CsA), azathioprine (AZA), mycophenolate mofetil (MMF), and rapamycin (RAPA). PD monitoring of CsA and MMF involves measuring the activity of the enzymes calcineurin and inosine monophosphate dehydrogenase, respectively. The PD of AZA is assessed by measuring the activity of thiopurine methyltransferase, which is induced by a metabolite of AZA, 6-mercaptopurine. The PD for RAPA involves measuring the activity of a P70 S6 kinase in lymphocytes. To date, the most detailed studies have been performed with PD monitoring of CsA and MMF. Similarities exist in the PD responses to CsA and MMF in renal-transplant patients. At trough concentrations in blood, both drugs reduce the activity of their target enzymes by only 50%; however, considerable interpatient variability is evident. Throughout the dosing interval, the enzyme activities parallel the respective drug concentrations. AZA treatment of renal-transplant patients who exhibited an increase in thiopurine methyltransferase activity from time of transplantation resulted in fewer episodes of active rejection. Additional clinical trials are currently underway to relate various pharmacokinetics and PD parameters to clinical response, to ascertain which provides the best guide for dosing. PD monitoring may provide an alternative approach to additional measurements of drug concentrations.

Comparison of binding characteristics of four rapamycin metabolites to the 14 and 52 kDa immunophilins with their pharmacologic activity measured by the mixed-lymphocyte culture assay

OBJECTIVES

To compare the binding characteristics of four rapamycin (RAPA) metabolites to the 14 and 52 kDa minor immunophilins with their pharmacologic activity, as measured by the mixed-lymphocyte culture (MLC) assay.

METHODS

Four RAPA metabolites were isolated by HPLC from the urine of renal transplant patients. Each metabolite was evaluated at 40 micrograms/L for its pharmacologic activity using the MLC assay. The results of the MLC assay were compared to those obtained using the radioreceptor assay (RRA), which measured the binding characteristics of equal concentrations of the metabolites to the 14 and 52 kDa minor immunophilins.

RESULTS

Each of the four RAPA metabolites showed low immunosuppressive activity by MLC. RM2 showed the highest activity, with 9% of parent RAPA activity. RM1, 3, and 4 showed 2%, 8%, and 4% activity, respectively. Only RM1 was found to bind significantly to either minor immunophilin, with 21% of parent binding to the 14 kDa protein and 25% of parent binding to the 52 kDa protein. RM2, 3, and 4 bound to both proteins with < or = 2% of parent binding.

CONCLUSION

We have demonstrated that the RRA for these four RAPA metabolites shows little cross-reactivity. There is no commercially available immunoassay for RAPA at present. The RRA, therefore, provides an excellent way to rapidly assess efficacy/toxicity of RAPA in patients receiving the drug.