Thymopentin: stability considerations and potency by various routes of administration.

Thymopentin, the synthetic pentapeptide Arg-Lys-Asp-Val-Tyr corresponding to amino acids 32-36 of thymopoietin, was rapidly degraded in human plasma (T 1/2 = 30 s) and this was shown to be due to proteolytic enzymes in plasma. The biological potency of thymopentin varied greatly with the route of administration, a finding possibly related to its short half-life. Intravenous infusion provided the most potent and bolus intraperitoneal injection the least potent mode of administration. Thus the route and rate of administration are critical factors in determining the effective dose being received by the animal.