A 2009 commentary reviewed the emerging use of aspirin in cardiovascular disease, specifically comparing oral dosing with rectal suppositories. The article highlighted a crossover study (see Table 1) in which 24 healthy volunteers received 162 mg oral aspirin (chewed) and 600 mg rectal suppositories, with salicylic acid levels measured at baseline, 30, 60, and 90 minutes. Results showed that both routes achieved systemic salicylic acid exposures within 90 minutes, indicating effective absorption. Peak aspirin levels occurred earlier with rectal dosing (as soon as 30 minutes) compared to oral dosing (60 minutes), suggesting relatively rapid systemic availability via the rectal route. Suppression of platelet aggregation was similar between the two routes, though some variability in absorption was noted. The commentary emphasized that rectal administration avoids first-pass hepatic metabolism, potentially allowing for earlier aspirin bioavailability. Based on these findings, the authors of the commentary suggest that stroke patients unable to take oral aspirin may potentially be candidates for an initial rectal aspirin suppository until an alternative feeding method is established. [1]
A dated 1975 investigation assessed the bioavailability of salicylate from different brands of commercially available aspirin rectal suppositories, each containing approximately 600 to 650 mg of aspirin. The study focused on adult subjects and highlighted the slow absorption profiles of these formulations compared to the oral administration of aspirin tablets. It was observed that, on average, at best, only about 40% of the administered dose was absorbed when the retention time in the bowel was limited to 2 hours. The research further noted that four out of the five products tested exhibited significantly lower absorption rates, averaging just around 20% bioavailability. The research utilized a cohort of adult male volunteers aged 24 to 35, who were instructed to insert the suppository in the morning and either evacuate their bowels after exactly 2 hours or retain the suppository for as long as possible beyond that, though retention times varied between 11 to >24 hours. Analytical methods involved collecting urine samples over a period of 28 to 32 hours and analyzing for total salicylate using a spectrophotometric method. The findings underscored that the brands coded L demonstrated the highest absorption rate compared to others (38%), yet absorption remained notably low unless retention times extended beyond 10 hours, revealing the substantial impact of retention time on drug availability. Despite the study's comprehensive comparative analysis and variation amongst different brands, the overall findings indicate that rectal suppositories delivered significantly reduced and slower aspirin absorption compared with oral administration, with only partial availability when retention time was restricted. [2]
A 1971 pharmacokinetic experiment examined the absorption of salicylates from rectal suppositories compared to oral administration. During the first phase of the study, twelve healthy adult subjects (10 male, 2 female), weighing 52 to 84 kg, received either aspirin 600 mg or sodium salicylate 534 mg orally upon arising. After 1 week, the subjects received suppositories containing the same doses of aspirin or sodium salicylate. Six different suppository bases incorporating aspirin and sodium salicylate were utilized. The subjects were ambulatory and permitted to eat and drink normally. Urine samples were collected every 2 hours after dosing and analyzed. Results showed that rectal administration of these drugs was equivalent to oral doses. However, the rate and extent of absorption were influenced by the choice of suppository base. Rapid release of aspirin was observed from suppositories made from polyethylene glycol and polyoxyethylene sorbitan monostearate with glyceryl monostearate, whereas aspirin release was suboptimal from sorbitan monopalmitate. Conversely, sodium salicylate demonstrated efficient release from a theobroma oil base. The polyethylene glycol suppositories released aspirin rapidly, yielding absorption profiles comparable to oral administration, while theobroma oil favored sodium salicylate release at body temperature, aligning with rapid systemic absorption. Overall, these findings underscore the importance of physicochemical properties and formulation factors in optimizing the rectal administration of analgesic drugs. [3]