Somatropin
Somatropin, a peptide. Recombinant human 22 kDa growth hormone (191 aa, two intrachain disulfides, identical to pituitary somatotropin).… Subcutaneous half-life 3.2 h.
Recombinant human 22 kDa growth hormone (191 aa, two intrachain disulfides, identical to pituitary somatotropin). Activates GHR (JAK2/STAT5) on hepatocytes → hepatic IGF-1 → systemic mitogenic / anabolic effects; direct effects at adipocytes (lipolysis) and myocytes. Plasma t½ ~2.6 h after SC injection; IGF-1 rise persists ~24 h, sustaining once-daily dosing. Long-acting derivatives (somapacitan, lonapegsomatropin) shift dosing to weekly.
PK: the SC block is re-sourced entirely. Its previous citation (PMID:33864240) is the right drug in the right species but its abstract is purely qualitative and states none of the four stored values. Bioavailability 0.495 is Laursen 1996 (PMID:8890721), 16 GH-deficient patients in an SC-versus-IV crossover, verbatim "The absolute bioavailability of GH following s.c. relative to i.v. administration was 49.5%" — the stored 0.75 was roughly 50% too high against the only measured human value. Half-life 3.23 h is Langbakke 2007 (PMID:17634080), verbatim "2.28 +/- 00.43 h vs. 3.23 +/- 00.75 h" for dialysis patients against 10 matched healthy individuals; the healthy figure is stored. ka 0.2 /h and V 28 L dropped — neither is established anywhere, and a time to peak of 3.5-4 h (PMID:12412826) contradicts that absorption rate. IV: Bright 1999 (PMID:10487702), 15 subjects under somatostatin suppression, verbatim "terminal half-life, 12.3 min" and "volume of distribution, 3.46 L". CAVEAT: that 3.46 L behaves as a CENTRAL compartment volume — the same abstract's clearance of 0.236 L/min with a 12.3 min half-life implies 4.19 L for a true one-compartment fit — so this route understates exposure duration. The IV ka of 60 /h was removed as an abstract-silent sentinel.
Pharmacokinetics
- Subcutaneous — t½ 3.2 h · F 50%
- IV — t½ 12 min · F 100% · Vd 3.46 L
Sources: PMID:33864240, PMID:8890721, PMID:17634080