Pharmacokinetic analysis during intravaginal progesterone-releasing devices (PRID®/CIDR®) application in the mare

Abstract

Exogenous administration of synthetic and natural progestin is commonly used in the mare during the annual transitional period to hasten the onset of the breeding season, as well as during the breeding season to suppress or synchronise oestrus and to maintain high blood progesterone concentration during pregnancy. Various routes of administrations were developed, one being the intra-vaginal insertion of a device, which allows continuous and prolonged absorption of progesterone. With the recent ban of oestradiol in cattle, "progestin-alo-ne" devices are the only products available in the cattle industry, two of which are PRID® and CIDR® (1,38g). The aim of this study was to investigate the pharmacokinetic parameters Cmax, tmax, AUCs and 1/2 from individual serum in mares with initially low progesterone serum concentration during an 8-day intra-vaginal administration of these two products. Similarities were found between the two devices with an adequate maintenance of high blood progesterone during administration. For both devices, the serum progesterone concentration pattern showed a rapid rise with a similar peak (Cmax((CIDR®) = 1 7.8 nmol/L and C max(PRID®) = 19.9 nmol/L) followed by a plateau and a rapid drop at device removal with concentrations < 3 nmol/L reached in less than 2 hours and an initial half-life (f/2) on the 0 to 120 minutes interval around 35 minutes. The AUC was found within the same range for both devices (1958 nmol/L/h and 2468 nmol/L/h for CIDR® and PRID®, respectively). It may hence be hypothesised that both intra-vaginal "progestin-alone" devices, CIDR® and PRID®, could be useful in the mare to enhance the first oestrous cycle during the transition period or to suppress oestrus or help in the maintenance of pregnancy in individual mares during the breeding season.