Enhanced rectal absorption of [Asu1,7]-eel calcitonin in rats using polyacrylic acid aqueous gel base

The effects of polyacrylic acid aqueous gel on the absorption of rectally administered [Asu1,7]-eel calcitonin, a calcitonin analogue, were investigated in rats. The [Asu1,7]-eel calcitonin (1 U/kg) was given into the rectal loop in gel bases at various pH (5.5-8.5) and polyacrylic acid concentrations (0.01-1.0% w/v). The maximum hypocalcemic effect was obtained in approximately 30 min after administration of the analogue in a 0.1% w/v polyacrylic acid gel base at pH 5.5. The plasma calcium level decreased by approximately 18% from the initial level. Rectal administration in vehicles such as polyethylene glycol 1000, triglyceride fatty acid mixture base, or saline solution had little or no hypocalcemic effect at a dose of 5 U/kg. The results indicated that a polyacrylic acid aqueous gel base significantly improved the absorption of this analogue. Furthermore, rectal administration in a polyacrylic acid gel base (0.1% w/v; pH 5.5) required a dose 35 times greater than an intravenously administered dose to achieve an equivalent hypocalcemic effect.