Brequinar sodium: monitoring immunosuppressive activity.

There has been remarkable consistency in our laboratory for the experimental results derived from a variety of allogeneic and xenogeneic models of organ transplantation using BQR, a primary immunosuppressive agent, to prevent graft rejection. Considerable knowledge exists in a number of species with respect to the dose-response immunosuppressive efficacy, organ specificity, peripheral drug measurements, toxic side-effects, species sensitivity, and synergistic drug interactions. Despite the complexity of transferring experimental results to the clinical setting, the cumulative experience with this new immunosuppressive compound suggests that it may be highly effective when used for clinical transplantation. The identification of a predictive monitoring technique(s) would provide clinicians with a highly sensitive marker to effectively regulate BQR posttransplantation immunosuppressive therapies. Although the maintenance of a "therapeutic window" of immunosuppressive efficacy did not necessarily prevent the rejection of allografts or xenografts in the rat models tested, there was an important correlation between elevated BQR levels and the onset of recipient toxicity. Increased morbidity and mortality was noted when BQR blood levels were elevated above 16 micrograms/mL. The measurement of pyrimidine biosynthetic pathway metabolites provided a direct correlation between graft viability, global depletion of nucleotides (UTP, CTP, ATP, GTP), and suppression of PHA-mitogenic lymphocyte proliferative responses when different doses of BQR were tested. The analysis of these parameters were less informative following combination therapy with CsA. As with other immunosuppressive agents, the difficulty in relying exclusively on plasma trough drug levels as an accurate means to project graft viability may be influenced by individual differences in absorption, metabolism, and excretion of the compound. Additional parameters that may provide valuable information for the posttransplant therapeutic course include drug bioavailability, peak levels, drug clearance, or inhibition of immune response. It is anticipated that these studies will provide important baseline information for further refinement of these monitoring techniques to aid in posttransplant patient management.