# Kisspeptin-54: The Longer Isoform in Clinical Studies

> Kisspeptin-54 (metastin) is the fifty-four-residue isoform — about 5857 Da, a ~28-minute half-life, the form used in the IVF trigger and amenorrhea trials. The longer burn, cited.

Fifty-four residues, a near-half-hour half-life, the isoform that carried the IVF and amenorrhea trials — the longer-burning signal, cited to source.

## The short version

Kisspeptin-54 is the long form of the kisspeptin family — fifty-four amino acids, originally called *metastin*. It does the same job as the short form (switching on the reproductive signal that drives LH and FSH), but it lasts much longer in the blood: around half an hour, against roughly four minutes for the short version. That staying power is why kisspeptin-54 is the form most of the human clinical studies used — including the IVF trials that triggered egg maturation with no severe over-stimulation, and the studies that coaxed lost menstrual cycles back. The same staying power has a catch: given continuously or twice daily, the effect fades within days as the receptor tires (tachyphylaxis). On this page, kisspeptin-54 is set against the shorter kisspeptin-10 so the trade-off — longer burn, but more prone to fading under continuous use — is clear. It is investigational; nothing here is a dose to follow.

## What kisspeptin-54 is

Kisspeptin-54 (KP-54) is the fifty-four-residue isoform cleaved from the KISS1 precursor, with a molecular weight of about `5857 Da` [8]. It was the first kisspeptin identified, originally named *metastin* when KISS1 was catalogued as a metastasis-suppressor gene — so 'metastin' and 'kisspeptin-54' name the same molecule. Like the shorter fragment it ends in the Arg-Phe-amide motif KISS1R requires and acts on the same receptor on hypothalamic GnRH neurons. The difference is size and durability: the larger peptide resists the plasma enzymes that quickly dismantle kisspeptin-10, so its signal burns longer.

## The pharmacokinetics: a near-half-hour burn

Kisspeptin-54's defining property is duration. Its measured human plasma half-life is `27.6 ± 1.1 minutes` — roughly seven times longer than kisspeptin-10's four-minute flare [8]. A mechanistic comparison found KP-54 reaches far higher peak plasma levels and sustains LH release substantially longer than KP-10 at equimolar doses, attributable to its greater resistance to peptidase cleavage, and that KP-54 also more effectively activated central GnRH neurons [11]. That longer burn is the reason the clinical trials leaned on this isoform: when the goal is a sustained, measurable gonadotropin response over an hour or more, the longer-lived peptide carries it.

## Kisspeptin-54 in the clinic: IVF triggering and restored cycles

Kisspeptin-54 carried the headline clinical work. In a Phase 2 randomized trial of 60 women at high risk of OHSS, a subcutaneous kisspeptin-54 bolus (`3.2–12.8 nmol/kg`) triggered oocyte maturation in 95% of women **with no case of moderate, severe or critical OHSS**, and the highest live-birth rate (62%) followed the `9.6 nmol/kg` dose — supporting kisspeptin-54 as a safer alternative trigger [5]. In women with hypothalamic amenorrhea, continuous IV kisspeptin-54 (`0.01–1.00 nmol/kg/h`) restored pulsatile LH secretion, lifting LH pulses about threefold and pulse secretory mass about sixfold versus vehicle, though the highest dose produced tachyphylaxis [4]. And the 2025 intranasal study delivered kisspeptin-54 by nasal spray (`12.8 nmol/kg`), stimulating LH across healthy men, healthy women and amenorrhea without adverse events — proof the long isoform can work without a needle [6]. The recurring caveat is the same one its durability creates: chronic or high-dose kisspeptin-54 desensitizes the axis, so the schedule matters as much as the dose [10].

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A noir reading of the Kisspeptin record — one upstream signal traced from KISS1 to GnRH and logged where the trials confirm it, the brief KP-10 flare set apart from the longer KP-54 burn, the fading-with-overuse and not-approved, not-a-supplement truths held in the light; no clinic behind the pulse, and nothing here dosed, dispensed, or sold.