Histrelin

Histrelin carries a benzylated D-histidine at position 6 and an ethylamide C-terminus, making it roughly a hundredfold more potent than native GnRH. The implant is a non-biodegradable hydrogel cylinder that releases about 50 to 65 micrograms per day over 12 months - a genuinely continuous exposure, which is precisely what maximises receptor downregulation. The trade-off against biodegradable depots is that the spent implant has to be surgically removed at the end of the year, and it can occasionally be difficult to retrieve if it has become encapsulated.

Targets: GnRH receptor (GnRHR), Pituitary gonadotrophs, LH, FSH, Testosterone, Oestradiol

• · The implant contains 50 mg and releases approximately 65 mcg per day. The old implant must be removed when the new one goes in.
• · Releases approximately 50 mcg per day. Vantas has been discontinued in some markets.

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• very commonImplant-site reaction— Bruising, soreness or a visible bump; usually settles in a week or two.
• very commonHot flushes— Prominent in adults on androgen deprivation, less so in children.
• commonHeadache
• commonFatigue
• commonBone mineral density loss— With long-term use; in children density generally recovers after treatment stops.
• commonInitial hormone flare— First few weeks after insertion.
• uncommonImplant extrusion or breakage on removal— A known procedural problem - fragments can be left behind and need imaging or a second procedure.
• rarePituitary apoplexy— In undiagnosed pituitary adenoma.

• Pregnancy.
• Known hypersensitivity to GnRH analogues.
• Inability to attend for implant removal at 12 months - leaving a spent non-biodegradable implant in place is not benign.

• conflictgonadorelin — Desensitises the same receptor.
• redundantleuprolide — Identical mechanism, different delivery vehicle.
• cautionAnticoagulants — Relevant to the insertion and removal procedures.

• · LH, FSH and sex steroids at 1 month and then periodically to confirm suppression.
• · Growth velocity and bone age in children.
• · Confirm the implant is palpable at each visit and mark the arm used, so removal at 12 months is straightforward.
• · Bone density with multi-year use.

Prescription drug in the US. Vantas has been discontinued in several markets, leaving Supprelin LA as the main product.

• Supprelin LA (histrelin acetate) subcutaneous implant prescribing information (label)
• Vantas (histrelin implant) prescribing information (label)

Histrelin takes the class principle to its logical conclusion. If continuous GnRH receptor occupancy is what produces suppression, then the ideal delivery system is one that never varies, and a hydrogel reservoir releasing about 65 micrograms a day for a year is as close to genuinely continuous as this pharmacology gets. There is no peak-and-trough, no monthly re-flare, and no window at the end of a dosing interval where levels drift and the axis stirs - which is a real phenomenon with monthly depots and a real reason some children escape suppression at the end of a cycle. That constancy is the therapeutic argument. The trade-off is mechanical rather than pharmacological: the implant is not biodegradable, so it has to come out, and retrieval after twelve months in the arm is occasionally difficult when fibrous encapsulation has formed. Implant breakage during removal, leaving fragments that need imaging and a second procedure, is a documented and irritating problem. In central precocious puberty the pharmacological endpoint is suppression of GnRH-stimulated peak LH below 4 mIU/mL, which the registration studies achieved in every treatment-naive subject at month 1 and maintained through month 12 - a completeness of response that is unusual and that reflects the delivery system rather than any special property of the peptide.

The problems here are surgical rather than pharmacological. The implant goes into the inner upper arm under local anaesthetic and has to come out a year later, and it does not always want to. Fibrous encapsulation makes retrieval difficult, the implant can break during removal, and fragments left behind need imaging and a second procedure. Extrusion through the skin happens occasionally. Beyond that, the failures are timing: an implant left in past twelve months stops delivering adequate drug and the axis begins to escape, which in a child means resumed bone age advancement and lost height potential that cannot be recovered. Replacement on schedule matters more than it feels like it should. Pituitary apoplexy after first exposure in someone with an undiagnosed adenoma is the rare catastrophic event described across this class.

• GnRH receptor (GnRHR) on pituitary gonadotrophs — Roughly a hundredfold the potency of native GnRH; a specific Kd was not resolved here.

  Continuous occupancy from the implant produces sustained receptor downregulation and complete gonadotropin suppression, without the interval-end drift seen with shorter depots.

• LH and FSH

  Brief initial rise after insertion, then suppression maintained for the full 12 months.

• Gonadal steroidogenesis (indirect)

  Prepubertal oestradiol or testosterone in children, castrate testosterone in adults with prostate cancer.

• Supprelin LA central precocious puberty registration studies (Study 1 and Study 2) 3 · n=47 · 52 weeks

  Suppression of GnRH-agonist-stimulated peak LH below 4 mIU/mL in children aged 3.7 to 11.6 years. Suppression was induced in all treatment-naive subjects and maintained in all pretreated subjects at month 1 and continued through month 12, with oestradiol suppressed in all 33 girls through month 9 and 97 percent at month 12.

Hormone suppression develops within 2 to 4 weeks of insertion after a brief initial flare in the first days. Suppression is then flat for the full twelve months, which is the entire design goal. In precocious puberty, clinical signs - breast development, growth velocity - regress over the following months, and bone age advancement slows over six to twelve months. On removal without replacement, the axis reactivates over weeks to months, and in children puberty resumes and proceeds normally, which is what makes this a genuinely reversible intervention.

There is very little to stack. In central precocious puberty, growth hormone is sometimes added in children with compromised height potential, which is a specialist decision and not a routine one. In adult prostate cancer use, the same antiandrogen flare cover applies at the start as with any agonist, and the same long-term bone and metabolic co-management. Calcium and vitamin D throughout is sensible in any multi-year suppression. Nothing in the GnRH stimulant direction - gonadorelin, kisspeptin - does anything against a continuously downregulated receptor, and combining another GnRH analogue is meaningless.

Against monthly or three-monthly depots for the same indication, the histrelin implant trades twelve injections a year for two minor procedures and delivers more consistent suppression, with no end-of-interval drift. For a child on multi-year treatment that is a substantial quality-of-life difference and the main reason it exists. Against nafarelin nasal spray in precocious puberty, it removes the adherence problem entirely, which is nafarelin's dominant failure mode. Against leuprolide, the pharmacology is equivalent and the choice is entirely about delivery preference, procedural access and cost. Its niche status is about price and the need for a minor operation rather than about how well it works, because on the evidence it works about as completely as anything in this class.

$1500–$3500/month. Supprelin LA has been reported at roughly 25000 to 40000 dollars per implant in the US, covering twelve months, which works out to a few thousand dollars a month before any payer negotiation. Vantas, the prostate cancer version, was considerably cheaper and has been discontinued in some markets. Figures are indicative and were not verified in this session.

• ?The three-letter sequence is reconstructed from the benzylated D-histidine and ethylamide substitutions described in the Core record rather than re-verified against a primary structural source in this session.
• ?Volume of distribution, clearance, protein binding and half-life are all explicitly absent from the label and were not sourced elsewhere. The 4-hour half-life carried in the Core record was not confirmed here.
• ?The time to steady state of 28 days is inferred from the observed hormonal suppression timeline rather than from a stated pharmacokinetic value.
• ?The registration study year is not given in the label section retrieved.
• ?Long-term outcome data on adult height after histrelin specifically, as opposed to GnRH agonist treatment generally, is limited by the small registration cohort of 47 children.
• ?Cost figures are indicative and were not verified in this session.

• SUPPRELIN LA (histrelin acetate) subcutaneous implant - FDA prescribing information DailyMed / FDA

  Source of the 65 mcg per day release rate, the 0.43 ng/mL median maximum serum concentration, and the two registration studies with 47 children between them. Also confirms the label does not report volume of distribution, clearance, protein binding or half-life.