Pegvisomant

Growth hormone signals by binding one receptor through site 1 and then recruiting a second receptor through site 2, causing functional dimerisation and JAK2-STAT5 activation. Pegvisomant is GH with eight substitutions that increase site 1 affinity and one critical G120K substitution that destroys site 2 binding, so it occupies receptors as a non-productive complex. Four to six 5 kDa PEG chains are attached, which extends the half-life to around six days and reduces immunogenicity but also lowers intrinsic receptor affinity, requiring the relatively high daily doses used clinically. Crucially it does not touch the pituitary adenoma at all: it normalises IGF-1 while serum GH stays high or rises, so GH is useless as a monitoring parameter and tumour size must be watched separately by MRI.

Targets: Growth hormone receptor site 2, JAK2-STAT5 signalling, Hepatic IGF-1 production

• · Titrate in 5 mg increments every 4 to 6 weeks based on IGF-1 alone. The label allows up to 30 mg daily; some centres go higher off-label.
• · Combination often allows lower pegvisomant doses than monotherapy, which matters given the cost. Weekly and twice-weekly regimens have been studied but are off-label.

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• commonElevated liver transaminases— Usually asymptomatic and often self-limiting, but ALT above 3 to 5 times the upper limit of normal requires investigation and possible discontinuation.
• commonInjection-site lipohypertrophy— Blocking GH receptors locally lets adipose tissue expand. Rotating sites is essential.
• commonHeadache and flu-like symptoms
• commonHypoglycaemia in diabetics— Removing GH's counter-regulatory effect improves insulin sensitivity; insulin and sulfonylurea doses often need reducing.
• commonAnti-GH antibodies— Detected in a substantial minority; rarely associated with loss of efficacy.
• uncommonTumour growth— The drug does not treat the adenoma, so pituitary MRI surveillance is mandatory.

• Hypersensitivity to pegvisomant or latex in the older diluent syringe
• Active liver disease or baseline transaminases above 3 times the upper limit of normal
• Caution in patients whose adenoma is already threatening the optic chiasm

• synergyoctreotide — Adding pegvisomant to a somatostatin analogue normalises IGF-1 in the large majority of partial responders and allows lower doses of each.
• synergylanreotide — Same combination logic as with octreotide.
• conflictsomatropin — Exact pharmacological opposites at the GH receptor.

• · Serum IGF-1 every 4 to 6 weeks during titration, then every 6 months
• · Liver function tests monthly for 6 months, then quarterly for the first year
• · Pituitary MRI at baseline, 6 months and then annually
• · Glucose in diabetic patients

Prescription drug in the US and EU. Also listed in this encyclopedia under growth hormone secretagogues, where it sits as the class antagonist.

• Somavert FDA prescribing information (label)
• Trainer et al. 2000 NEJM, randomised trial of pegvisomant in acromegaly (trial)
• ACROSTUDY long-term safety surveillance registry reports (other)

Growth hormone signalling requires sequential engagement of two receptor molecules by a single ligand. Site 1 on the hormone binds the first receptor with high affinity; site 2 then recruits a second receptor into a functional dimer, which juxtaposes the associated JAK2 molecules so they trans-phosphorylate, recruit and phosphorylate STAT5b, and drive transcription of IGF-1 and its binding proteins in hepatocytes. Pegvisomant exploits the asymmetry: eight substitutions make site 1 bind tighter, and the single G120K substitution makes site 2 unable to recruit the second receptor. The result is a stable, non-productive 1:1 complex that occupies the receptor and blocks productive dimerisation. Two consequences follow that define how the drug is used. First, it acts entirely at the periphery, principally in the liver, and it does nothing whatsoever to the pituitary adenoma, so the tumour is untouched and must be watched separately by MRI. Second, because it blocks IGF-1 production, it removes the negative feedback IGF-1 exerts on pituitary GH secretion, so serum GH rises on treatment. Any attempt to titrate on GH will therefore lead you in exactly the wrong direction, and the assay problem compounds this: pegvisomant cross-reacts with many GH immunoassays, so the number you get is partly the drug. IGF-1 is the only valid titration parameter. The lipohypertrophy at injection sites is elegant confirmation of local mechanism: blocking GH receptors in the subcutaneous fat around the needle removes GH's lipolytic tone and the fat expands there specifically.

The signature error is titrating on growth hormone. GH rises on this drug because IGF-1 feedback is gone, and it cross-reacts with many GH assays, so a clinician watching GH will conclude the drug is failing and escalate a patient who is already over-suppressed. IGF-1 only. The second is forgetting the tumour: pegvisomant is a peripheral blocker, the adenoma is untreated, and there are documented cases of tumour growth in patients whose IGF-1 was beautifully controlled. MRI at baseline, 6 months and annually is not optional. The third is impatience during titration, because a 6-day half-life means steady state takes 4 to 6 weeks and an IGF-1 checked at 2 weeks will underestimate the effect and prompt an unnecessary increase. The fourth is reconstitution technique: shaking the vial foams and denatures the protein, and the reconstituted solution contains no preservative so it is single-use within 6 hours. The fifth is lipohypertrophy from failing to rotate sites, which is entirely predictable from the mechanism and entirely preventable. The sixth is over-suppression, which produces a functional GH-deficient state that patients describe as flat, fatigued and unwell, and which is fixed by reducing the dose rather than adding anything.

1. 40 mgDay 1 — 40 mg loading dose subcutaneously under medical supervision. Reconstitute with the 1 mL of sterile water supplied, injecting down the vial wall and rolling gently between the palms. Do not shake: the protein foams and denatures.
2. 10 mgFrom day 2 — 10 mg once daily. Do not adjust for 4 to 6 weeks; steady state takes that long and earlier IGF-1 readings are misleading.
3. 15 mgWeeks 5-10 — Increase in 5 mg increments every 4 to 6 weeks, guided strictly by IGF-1.
4. 20 mgContinuing titration — 20 mg daily. Most patients achieve IGF-1 normalisation somewhere between 10 and 20 mg.
5. 30 mgUpper licensed dose — 30 mg daily is the label maximum. Some specialist centres go higher off-label in genuine non-responders, and in combination with a somatostatin analogue lower total doses often suffice.

• Growth hormone receptor, site 2 interaction — Site 1 affinity is increased by eight substitutions; site 2 binding is abolished by G120K. PEGylation reduces net affinity, which is why doses are in the tens of milligrams.

  Blocks functional receptor dimerisation, preventing JAK2 trans-phosphorylation and STAT5b activation. Hepatic IGF-1 transcription falls.

• JAK2-STAT5b signalling in hepatocytes

  Suppressed, which is the mechanism of IGF-1 normalisation and of the improved insulin sensitivity that frequently causes hypoglycaemia in diabetic patients.

• Growth hormone receptor on subcutaneous adipocytes — Same receptor, local effect

  Local blockade removes GH's lipolytic tone, producing injection-site lipohypertrophy. Site rotation is not cosmetic advice, it is mechanistic necessity.

• Pituitary somatotroph adenoma — None

  No effect at all. GH secretion rises because IGF-1 feedback is removed, and the tumour requires independent MRI surveillance.

• Trainer 2000 randomised trial of pegvisomant in acromegaly Randomised, double-blind, placebo-controlled, dose-ranging · 2000

  Change in serum IGF-1 in patients with acromegaly. Pegvisomant produced dose-dependent IGF-1 reduction with normalisation in the majority at higher doses, establishing growth hormone receptor antagonism as a viable therapeutic strategy.

• van der Lely 2001 long-term pegvisomant study Open-label extension · 2001

  Long-term IGF-1 normalisation with pegvisomant. IGF-1 normalised in around 90 percent of patients at adequate doses, with tumour size monitored separately.

IGF-1 begins falling within the first two weeks, but the concentration-effect relationship is slow: peak serum levels are not reached for 33 to 77 hours after a single dose, the half-life is around 6 days, and steady state takes 4 to 6 weeks of daily dosing. That is why dose decisions are made at 4 to 6 week intervals and not sooner. Soft tissue swelling, sweating, headache and joint symptoms typically improve over the first one to three months. Liver enzyme elevations, when they occur, cluster in the first 6 months, which is why monitoring is monthly then. Stopping causes IGF-1 to climb back within weeks.

Combining pegvisomant with octreotide LAR or lanreotide is the standard second-line construction in acromegaly and it is genuinely rational: the somatostatin analogue reduces GH output and gives some tumour control, pegvisomant blocks whatever GH escapes from acting on the liver. In practice the combination normalises IGF-1 in the large majority of somatostatin partial responders and does so at lower pegvisomant doses than monotherapy, which matters enormously given the cost. Weekly and twice-weekly pegvisomant regimens on top of a depot analogue have been studied and are used off-label for exactly that reason. Cabergoline is a cheap third agent worth adding before escalating pegvisomant. Do not combine with somatropin, which is the exact pharmacological opposite. Watch the diabetes regimen closely: improved insulin sensitivity means insulin and sulfonylurea doses usually come down, sometimes substantially. If the patient has had radiotherapy, remember that its effect accumulates over years and pegvisomant requirements may fall.

Against somatostatin analogues: pegvisomant normalises IGF-1 far more reliably, around 90 percent versus perhaps half for first-generation analogues, but it treats only the consequences of GH excess and leaves the tumour entirely alone. Somatostatin analogues shrink or stabilise adenomas; pegvisomant does not. That distinction, not efficacy, is what determines which is first-line. Against pasireotide: pasireotide controls more patients than octreotide and does act on the tumour, but at the price of hyperglycaemia in most users, whereas pegvisomant improves glucose handling. In a diabetic acromegalic who has failed octreotide, that contrast is decisive. Against surgery and radiotherapy: transsphenoidal surgery remains first-line and is the only route to cure; radiotherapy works slowly over years and carries hypopituitarism risk. Pegvisomant is what holds the biochemistry while those play out or after they have failed. Against somatropin: exact opposites at the same receptor, which is a useful way to remember that this drug is a growth hormone molecule doing the reverse of its job.

$6000–$20000/month. Unverified estimate of US pricing, and it scales directly with dose: a patient on 30 mg daily costs roughly three times one on 10 mg. This cost is the main reason combination therapy with a somatostatin analogue, which lowers the required pegvisomant dose, is used so widely. Not sourced in this session.

• ?Plasma protein binding is not stated in the label.
• ?The 55 hour tmax is the midpoint of the label's 33 to 77 hour range.
• ?The 35-day time to steady state is the midpoint of the conventional 4 to 6 week figure rather than a directly quoted label number; the label gives a half-life range of 60 to 138 hours.
• ?Neither pivotal trial's enrolment nor duration was confirmed against the papers, so both are null.
• ?The exact frequency of clinically significant tumour growth on pegvisomant remains debated; ACROSTUDY registry data are reassuring but registry data have their own limitations, and I did not verify a specific ACROSTUDY publication this session.
• ?Cost figures are unverified estimates.

• Treatment of acromegaly with the growth hormone-receptor antagonist pegvisomant Trainer PJ, et al., New England Journal of Medicine, 2000 · PMID 10770982

  The landmark trial that proved receptor antagonism works in acromegaly.

• Long-term treatment of acromegaly with pegvisomant, a growth hormone receptor antagonist van der Lely AJ, et al., Lancet, 2001 · PMID 11734231

  The long-term data showing IGF-1 normalisation in around 90 percent, which is why this remains the most reliably effective biochemical control agent in acromegaly.

• SOMAVERT (pegvisomant) for injection - FDA prescribing information Pfizer, DailyMed

  Source of the 33 to 77 hour tmax, 57 percent relative absorption, 7 L apparent volume of distribution, 28 to 36 mL/h clearance, 60 to 138 hour half-life range, under 1 percent urinary excretion, and the 191-residue backbone with 42, 47 and 52 kDa PEG variants.