The Effects of Chronic Subcutaneous Administration of an Investigational Kisspeptin Analog, TAK-683, on Gonadotropin-Releasing Hormone Pulse Generator Activity in Goats

Abstract

The continuous activation of the kisspeptin receptor by its agonists causes the abrogation of kisspeptin signaling, leading to decreased pulsatile luteinizing hormone (LH) secretion. Employing this phenomenon as a tool for probing kisspeptin action, this study aimed to clarify the role of kisspeptin in gonadotropin-releasing hormone (GnRH) pulse generation in goats. We examined the effects of chronic administration of TAK-683, an investigational kisspeptin analog, on LH secretion, GnRH immunostaining, pituitary responses to exogenous GnRH, and GnRH pulse generator activity, reflected by a characteristic increase in multiple-unit activity (MUA volley). An osmotic pump containing TAK-683 was subcutaneously implanted on day 0. TAK-683 treatment dose-dependently suppressed pulsatile LH secretion on day 1. Higher doses of chronic TAK-683 profoundly suppressed pulsatile LH secretion but had little effect on GnRH immunostaining patterns and pituitary responses to GnRH on day 5. In ovariectomized goats, MUA volleys occurred at approximately every 30 min on day -1. On day 5 of chronic TAK-683 administration, pulsatile LH secretion was markedly suppressed, whereas MUA volleys were similar to those observed on day -1. Male pheromones and senktide (neurokinin B receptor agonist) induced an MUA volley but had no effect on LH secretion during chronic TAK-683 administration. The results indicate that the chronic administration of a kisspeptin analog profoundly suppresses pulsatile LH secretion without affecting GnRH content, pituitary function or GnRH pulse generator activity, and they suggest an indispensable role for kisspeptin signaling in the cascade driving GnRH/LH pulses by the GnRH pulse generator.

Mechanistic insights into the more potent effect of KP-54 compared to KP-10 in vivo

“…The ability of KP-10 to cross the BBB is an important question as several KP-10 analogues have been developed to act as GPR54 agonists for manipulation of the reproductive axis and to treat clinical conditions including prostate cancer [34–42]. One of these KP-10 analogues (C6) has been shown to induce synchronized ovulations and allow pregnancies after a single intramuscular injection in sheep [36].…”

KISS1R: Hallmarks of an Effective Regulator of the Neuroendocrine Axis

“…Over the last decade, studies conducted on mice, rats, sheep, monkeys, and humans have examined the desensitization of KP signaling following chronic exposure to KP (summarized in table 1) [68,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113] and, while most studies detected desensitization during the course of investigation, the temporal aspect of desensitization was surprisingly widely variable, ranging from 1-2 h to greater than 24 h. Based on several studies investigating the desensitization of GPCRs, including KISS1R, there is evidence strongly suggesting that the range of temporal responses following chronic exposure of a receptor-expressing system to its agonist is in part due to differences in ligand efficacy and concentration [108,109,110,111,112,113,114] as well as differences in developmental (which would reflect differences in circulating levels of gonadal steroids) and metabolic status [103]. …”

“…While the dose-response study by Plant and colleagues [102] could not establish a relationship between KP concentration and the rate of KISS1R desensitization, several studies using synthetic KP analogs would subsequently do so (table 1) [108,110,111,112,113]. For example, in a recent study from the Okamura laboratory [113], the authors examined the effects of chronically administered KP agonist TAK-683 (an investigational protease-resistant agonist which exhibits improved pharmacokinetics and higher affinities than natural ligands) on LH secretion in goats. An osmotic pump containing TAK-683 (0.03, 0.3, or 3 nmol/h/kg) was subcutaneously implanted (day 0) and TAK-683 was observed to suppress LH secretion as early as on day 1 and with marked suppression on day 5.…”

“…In healthy human males, continuous s.c. infusion of another KP analog, i.e. TAK-683 (which has properties similar to those of TAK-448), triggered subcastrate levels of testosterone by day 7, while in goats s.c. continuous infusion of TAK-683 suppressed LH secretion after as little as 1 day, with marked suppression by day 3 (table 1) [110,113]. Because of the remarkable stability of the synthetic agonists in serum, just a single daily s.c. injection of TAK-448 and TAK-683 was sufficient to achieve desensitization of LH and testosterone secretion in intact adult male rats by day 4 (table 1) [111].…”

“…It is important to note that while the above studies clearly demonstrate the ability to downregulate the neuroendocrine-reproductive axis through chronic exposure to high-affinity and more stable KP agonists, either as continuous infusions or as single daily injections, these studies also clearly reveal that the kinetics of KISS1R desensitization can be modulated by ligand affinity, efficacy, stability, and concentration [108,110,111,112,113,114]. Therefore, as we continue to develop KP-based therapies, attention must be paid to ligand delivery (continuous vs. pulsatile), efficacy, and concentration.…”