Valorphin is an endogenous hemoglobin β-chain (33-39) fragment with opioid analgesic activity, binds to rat mu-opioid receptor, with an IC₅₀ of 14 nM; Valorphin also shows anti-tumor activity. IC50 & Target: IC50: 14 nM (mu-opioid receptor), 200 nM (δ-opioid receptor)^[1] In Vitro: Valorphin is a derivative of dihydrovaltrate with opioid analgesic activity, binds to rat mu-opioid receptor, with an IC₅₀ of 14 nM. Valorphin has low affinity for δ-opioid receptor (IC₅₀, 200 nM) and shows no affinity for κ receptor (IC₅₀, >10 μM). Valorphin (>10 μM) decreases spontaneous firing rate of cerebellar rat Purkinje cells^[1]. Valorphin (1 μM) treatment 48 h prior to 0.1 μM epirubicin, or 0.1 μM vincristine, or 0.05 μM vincristine, causes 100% tumor cell death^[2]. In Vivo: Valorphin exhibits pronounced analgesic activity in mice, rats and rhesus monkeys via s.c, with ED₅₀s of ≤5.2 mg/kg, but barely active after oral administration^[1]. Valorphin (1 mg/kg) causes 42% of tumor growth inhibition in female BLRB mice bearing syngeneic mammary carcinoma cells^[2].