Stable prodrugs of n-butyric acid:: suppression of T cell alloresponses in vitro and prolongation of heart allograft survival in a fully allogeneic rat strain combination

n-Butyric acid has previously been shown in vitro to suppress T cell alloresponses and beyond that to induce a state of alloantigen-specific hyporesponsiveness suggesting a potential relevance for suppressing alloresponses also in vivo. The clinical use of butyrate salt derivatives, however, is limited by an extremely short half-life due to rapid metabolism. This prompted us to investigate the effect of butyric acid derivatives with prolonged residence time in vivo on T cell alloresponses in vitro and further to explore the immunosuppressive capacity of esterified n-butyric acid in vivo. First, the effect of three butyric acid esters, i.e. glucose pentabutyrate, diacetone glucose butyrate and tributyrin on T cell proliferation in a human mixed lymphocyte culture (MLC) was evaluated. All three derivatives were found to inhibit T cell alloresponses in a concentration-dependent manner. Based on the ED50 values, glucose pentabutyrate was found to be most effective in inhibiting T cell alloreactivity in vitro (11 mu M), followed by diacetone glucose butyrate (122 mu M), tributyrin (146 mu M) and sodium butyrate (539 mu M). Because of its favourable in vitro properties, glucose pentabutyrate was chosen for in vivo experiments. To test the effect of this compound on allograft survival in vivo, in the second part of this study, heterotopic heart transplants were performed in a high responder fully allogeneic rat strain combination (Brown Norway to Lewis strain rats). We found that intraperitoneal (i.p.) injection of glucose pentabutyrate at 500 mg/kg/day (day 0 and daily up to 12 days post transplant) induced a significant prolongation of allograft survival as compared to animals treated with vehicle (glycerol formal, i.p.) alone (14.1 +/- 6.3 versus 9.6 +/- 3.2 days, p = 0.036), whereby at lower dosage (100 mg/kg/day) no such effect was observed (10.2 +/- 2.1 days, p = 0.21). Our findings suggest that stable prodrugs of n-butyric acid might have potential clinical relevance for inhibiting alloresponses in vivo.