Effect of Abdominal Radiation Therapy on Drug Absorption in Humans

The absorption of oral digoxin and of desmethyldiazepam, from its precursor clorazepate, was studied in seven patients who had received abdominal and/or pelvic radiation therapy for neoplastic disease. All patients were in remission and had normal renal function and no evidence of malabsorption. Single 0.5-mg doses of digoxin tablets and 15-mg doses of clorazepate were administered in the fasting state. Concentrations of digoxin (by radioimmunoassay) and desmethyldiazepam (by gas chromatography) were determined in multiple plasma samples and all urine collected during 24 hours after dosage. The mean (+/- S.E.) weight-normalized area under the 24-hour plasma digoxin concentration curve (WtN-AUC-24) in the patients (722 +/- 40 ng/ml-hr-kg) was similar to that in five normal controls (713 +/- 57 ng/ml-hr-kg), but 24-hour urinary excretion of digoxin in patients (54.5 +/- 4.4 microgram) was significantly less (P less than 0.025) than in controls (83.4 +/- 11.4 microgram). Neither age, sex, nor renal function explained the difference. In the clorazepate study, WtN-AUC-24 for desmethyldiazepam in the patients (187 +/- 19 microgram/ml-hr-kg) was significantly less (P less than 0.01) than in 15 normal control subjects (230 +/- 5 microgram/ml-hr-kg). Age and sex did not explain the difference. Thus, radiation therapy, or the underlying disease, is associated with malabsorption of these two drugs, possibly because of damage to gastric acid-secreting cells.