Systemic levels of glyceryl trinitrate following topical application to the anoderm do not correlate with the measured reduction in anal pressure

Background. Topical 0.2 per cent glyceryl trinitrate (GTN) lowers resting anal pressure (RAP) and heals two-thirds of chronic anal fissures. Over 50 per cent of patients experience headache, presumably through systemic absorption, but the pharmacokinetics; of GTN ointment are unknown. This study evaluated the systemic absorption profile of GTN, and correlation between plasma GTN levels, RAP, haemodynamic variables and side-effects. Methods: Thirty healthy volunteers were recruited with local medical school ethics committee approval. Continuous static anal manometry was performed for 10 min before and 2 h after application of 0.2 per cent GTN (0-5 g) to the anoderm. Blood samples were taken from an intravenous cannula, and pulse and blood pressure were measured before application of GTN, and 10, 20, 30, 40, 50, 60, 90, 120 and 180 min thereafter. Details of side-effects were recorded. Results: GTN was detected in the plasma 10 min to 3 h after topical application. RAP was significantly reduced after 10 rain, but had returned to pretreatment values by 120 min. Pulse was statistically unchanged during the study; systolic blood pressure was significantly lower 20-90 min after GTN application, and diastolic pressure was decreased throughout the study. Headaches were experienced by 14 of 30 volunteers after a median (range) of 41 (4-120) min, persisting for 74 (30-176) min, with an intensity score of 19 (5-30) mm represented on a 100-mm linear visual analogue scale. There was no correlation between plasma GTN concentration, RAP, and the onset, duration or intensity of headaches. Conclusion: Topical GTN acts locally on the internal anal sphincter; systemic levels do not contribute significantly to the reduction in RAP. There is marked interindividual variation in plasma GTN levels, and no correlation with haemodynamic variables and side-effects.