DIALYSIS OF THE RECTUM FOR SAMPLING DRUG CONCENTRATIONS IN THE LUMINAL EXTRACELLULAR FLUID OF THE GUT - TECHNIQUE AND PRECISION

Background: It is useful to measure the luminal concentration of drugs which act in the gut. Dialysis of the rectum has not previously been used or validated for this purpose. Aim: To determine the precision of rectal dialysis for measuring rectal drug concentrations. Methods: To establish the duration of dialysis required to approach equilibrium, the rate of methotrexate diffusion into dialysis bags was first determ ined in vitro. The precision of rectal dialysis for sampling the metho trexate concentration of colonic lumen extracellular fluid was determi ned in seven subjects who underwent two consecutive dialysis procedure s. Subjects treated with subcutaneous methotrexate for refractory infl ammatory bowel disease were studied. Results: Methotrexate crossed the dialysis membrane by a first-order process, and after a 2 h in vitro dialysis, equilibration was 74 +/- 2% (mean +/- s.d.) complete. Rectal dialysis was well tolerated by all subjects. The mean+/-s.e. methotre xate concentration of 3.6 +/- 1.1 nmol/L in the first dialysate was no t significantly different from 3.6 +/- 0.9 nmol/L in the second dialys ate, P = 0.99 (paired two-tailed t-test). Similar precision was obtain ed for an endogenous molecule, potassium, secreted by the rectal mucos a. Conclusions: Dialysis of the rectum is a well tolerated and precise technique for sampling the colonic lumen extracellular fluid for quan titative analyses of exogenous and endogenous substances.