Ac-SDKP vs Met-Enkephalin

Inhibits hematopoietic stem cell entry into S-phase. Blocks TGF-β1-mediated fibroblast activation, reducing collagen deposition. Promotes angiogenesis via VEGF upregulation. Regulated in vivo by ACE enzyme.

Ac-SDKP (N-acetyl-seryl-aspartyl-lysyl-proline) is an endogenous tetrapeptide generated from the N-terminus of thymosin beta-4 by the enzyme prolyl oligopeptidase, with further regulation by angiotensin-converting enzyme (ACE), and is characterized by roles in hematopoietic progenitor regulation and anti-fibrotic signaling in renal, cardiac, and vascular tissue. Ac-SDKP inhibits collagen synthesis and fibroblast proliferation, reduces TGF-beta-1-mediated fibrotic signaling, and promotes anti-inflammatory macrophage polarization in preclinical models, suggesting a role in tissue homeostasis downstream of the thymosin beta-4 pathway. Preclinical studies in rodent models of renal fibrosis and systemic lupus erythematosus — predominantly from the NIH-funded Rhaleb and Carretero laboratory at Henry Ford Health — have demonstrated that exogenous Ac-SDKP reduces collagen deposition and inflammatory infiltrate; no human clinical trials have been completed or indexed in PubMed. Ac-SDKP has no FDA approval and no approved indication in any jurisdiction; it is studied as a research compound with a plausible anti-fibrotic mechanism and consistent preclinical evidence, but extrapolation of rodent findings to human therapeutic outcomes has not been validated by any clinical investigation.

Binds OGF receptors (zeta opioid receptors) on cell nuclei, regulating DNA synthesis and cell proliferation in a tonic inhibitory manner. Also binds delta opioid receptors. Intermittent blockade by low-dose naltrexone upregulates receptor expression and endogenous MENK tone.

Methionine enkephalin (Met-enkephalin; opioid growth factor, OGF) is an endogenous pentapeptide (Tyr-Gly-Gly-Phe-Met) that acts at classical mu- and delta-opioid receptors for pain modulation and, in its OGF role, functions as a tonic inhibitor of cell proliferation through interactions with the nuclear OGF receptor (OGFr, the zeta-opioid receptor), a pathway with distinct biological implications from classical analgesic opioid signaling. The OGF-OGFr axis operates as an endogenous homeostatic brake on cell cycle progression: OGF binds OGFr in the nucleus to upregulate cyclin-dependent kinase inhibitors p16 and p21, inhibiting S-phase entry; this mechanism has been characterized in models of wound repair, organ development, and immune cell regulation. Research from the Zagon laboratory has described the OGF-OGFr axis as a broad homeostatic regulator of cell proliferation and has established that the duration of opioid receptor blockade determines biotherapeutic response magnitude — the mechanistic basis for proposed low-dose naltrexone-mediated OGF upregulation in inflammatory and neoplastic conditions. Methionine enkephalin has no FDA-approved therapeutic applications as an exogenous agent; the OGF pathway has been characterized through preclinical and early-stage research, and exogenous OGF administration for any indication has not established a human clinical evidence base or received regulatory approval.