Acyline

Summary

Acyline is a synthetic peptide that acts as a gonadotropin‑releasing hormone (GnRH) antagonist. It is being investigated for its ability to suppress the pituitary release of luteinising hormone (LH) and follicle‑stimulating hormone (FSH), thereby lowering circulating sex steroids such as testosterone. Studies in dogs and healthy men have shown that a single subcutaneous dose can produce reversible hormonal suppression for several days, and an oral formulation using a GIPET enhancer has demonstrated short‑term suppression of testosterone in men.

Mechanism of Action

Acyline binds competitively to the GnRH receptor on pituitary gonadotrophs without activating it, blocking the normal pulsatile GnRH signal. This antagonism prevents the synthesis and secretion of LH and FSH, which in turn reduces downstream gonadal steroid production, chiefly testosterone in males and estradiol in females. The peptide’s high affinity and resistance to rapid degradation allow sustained receptor occupancy after a single dose, leading to prolonged hormone suppression.

What the Research Shows

In male dogs, a single subcutaneous dose of 330 µg/kg acyline lowered serum LH, FSH and testosterone within an hour, with nadir values reached by day 9 and gradual return to baseline by day 14–29; no local or systemic adverse effects were observed. Female dogs given 110 or 330 µg/kg acyline experienced interruption of the estrous cycle, absence of ovulation and a reversible delay of the next cycle, again without detectable side effects. In healthy men, subcutaneous injections ranging from 2.5 to 75 µg/kg produced dose‑dependent suppression of LH, FSH and testosterone lasting over 48 h, with only mild, transient injection‑site reactions. An oral GIPET‑enhanced tablet (10–40 mg) achieved significant LH, FSH and testosterone suppression after 6–12 h, returning to baseline by 48 h, and was well tolerated. A separate study highlighted the potential for depot‑type formulations to extend suppression. Across studies, the hormone‑lowering effect was reversible and safety signals were minimal, though sample sizes were small and follow‑up limited.

Reported Benefits

Acyline reliably and reversibly suppresses gonadotropins and sex steroids, offering a tool for temporary contraception, control of canine reproductive cycles, and potential adjunct therapy for hormone‑dependent conditions such as prostate cancer. The ability to achieve suppression with a single injection and the experimental oral formulation suggest flexibility in administration compared with existing GnRH antagonists that require frequent dosing or depot injections.

Limitations of the Evidence

Evidence is limited to short‑term studies in a small number of dogs and healthy men; long‑term efficacy, safety, and effects on fertility have not been established. No regulatory approval exists for any indication, and the oral GIPET formulation remains investigational. Data on dose optimisation, chronic use, and outcomes in disease states such as prostate cancer are lacking, making clinical translation premature.

Safety Considerations

In canine studies, no hematological, biochemical, local, or systemic adverse events were reported after a single high‑dose injection. Human participants experienced only occasional mild injection‑site blush or pruritus that resolved within 90 minutes, and the oral study reported no serious adverse events or laboratory abnormalities. Potential concerns include transient hypogonadal symptoms due to testosterone suppression and unknown risks with repeated or prolonged dosing.

How It Is Administered

Acyline has been administered subcutaneously in studies, typically as a single dose calculated in micrograms per kilogram body weight. An experimental oral tablet formulation using the GIPET enhancer has been tested at fixed milligram doses (10–40 mg) and shown to achieve systemic exposure. Both routes are investigational and not yet approved for clinical use.

Routes of Administration

Goals & Uses

• Precocious puberty treatment (research)Pediatric EndocrinologyLow
• Prostate cancer therapyOncologyLow
• Research tool for HPG axis studiesResearchHigh
• Endometriosis treatmentGynecologyLow
• Prostate cancer androgen suppressionOncologyModerate
• Testosterone suppressionOncology/EndocrinologyModerate
• Male contraceptionReproductive EndocrinologyModerate
• Suppression of gonadotropins (LH/FSH)EndocrinologyModerate

Contraindications

• known hypersensitivity to peptideImmunologicModerate
• Severe hepatic impairmentOrganModerateLiver function concerns
• PregnancyPopulationHighPotential fetal risk or insufficient safety data
• Hypersensitivity to GnRH analoguesAllergy/ImmunologyHigh
• Osteoporosis or significant bone lossMusculoskeletalModerate

Adverse Effects

• Mood changesPsychiatricUncommon
• Injection site reactionsLocalCommon
• Injection site painLocalCommonPain at the injection site
• HeadacheNeurologicUncommonPain in the head or upper neck
• Decreased libidoSexual / EndocrineUncommon
• Hot flashesEndocrine / VasomotorCommon
• Hypogonadism symptoms (hot flashes, reduced libido)Endocrine/ReproductiveCommon
• Bone mineral density reductionMusculoskeletalUncommon

Drug Interactions

• Testosterone replacement therapyModerate
• CYP3A4 inhibitorsLow
• Estrogen-containing contraceptivesModerate
• CYP450 inhibitorsLow
• Other GnRH analoguesModerate
• Testosterone (exogenous)Low

Population Constraints

• Women of child‑bearing potentialGenderAbsolute
• Pediatric patientsAgeAbsolute
• ElderlyAgeRelative
• Patients with pre-existing cardiovascular diseaseCardiovascularRelative
• Postmenopausal womenReproductive StatusRelative

Regulatory Status

• European UnionInvestigationalClinical studies ongoing; no EMA marketing authorization.
• United StatesInvestigationalPhase II trials completed; not FDA‑approved.
• United KingdomInvestigationalLimited trial use; not licensed.

Investigational product; no FDA, EMA, or MHRA approval. Clinical trials have evaluated safety and efficacy.

Evidence & Sources

• Endocrine effects of the GnRH antagonist, acyline, in domestic dogs
  Journal ArticleModerateGarcía Romero G, et al.2009-01-01T00:00:00.000000Z
• Delivery of Peptidic Gonadotropin Releasing Hormone Antagonists
  Journal ArticleModerateAlSheyyab RY, et al.2018-01-01T00:00:00.000000Z
• Interruption of the canine estrous cycle with a low and a high dose of the GnRH antagonist, acyline
  Journal ArticleModerateValiente C, et al.2009-01-01T00:00:00.000000Z
• Effect of the GnRH antagonist, acyline, on canine testicular characteristics
  Journal ArticleModerateValiente C, et al.2007-01-01T00:00:00.000000Z
• Acyline: the first study in humans of a potent, new gonadotropin-releasing hormone antagonist
  Journal ArticleModerateHerbst KL, et al.2002-01-01T00:00:00.000000Z
• Oral administration of the GnRH antagonist acyline, in a GIPET-enhanced tablet form, acutely suppresses serum testosterone in normal men: single-dose pharmacokinetics and pharmacodynamics
  Journal ArticleModerateAmory JK, et al.2009-01-01T00:00:00.000000Z

Frequently Asked Questions