INTEGRATED PHARMACOLOGICAL AND PHARMACOKINETIC PROFILING REVEALS DISTINCT PROPERTIES OF METHADONE AND W-15

Opioid drugs induce rapid tolerance and strong psychological and physical dependence, contributing substantially to the global drug abuse burden. Methadone and W-15 are strictly regulated opioids in several European countries, including Germany and Sweden, yet integrated comparative analyses of their pharmacological properties remain limited. Here, we conducted integrated pharmacological and pharmacokinetic profiling to characterize drug-specific differences between the two compounds. Methadone exhibited high affinity for the mu-opioid receptor (K_i = 0.3 nM), whereas W-15 showed lower affinity (K_i = 5.0 nM). Furthermore, Western blot analyses revealed distinct signaling profiles in ventral tegmental area (VTA), with methadone reducing mu-opioid receptor expression, beta-arrestin2, adenylyl cyclase 1 (AC1), p-JNK, and p-ERK/ERK, whereas W-15 selectively decreased protein kinase A (PKA) and p-p38. Consistent with these molecular differences, nucleus accumbens (NAc) dopamine levels were increased following methadone but not W-15 treatment. Validated LC–MS/MS analysis revealed distinct pharmacokinetic profiles. Methadone showed higher systemic clearance (approximately 110–157 mL/min/kg) and larger volume of distribution compared with W-15 (approximately 71–80 mL/min/kg). Brain distribution analysis showed that methadone readily penetrated the blood–brain barrier, with brain-to-plasma partition coefficients (K_p) consistently exceeding 1 (AUC_last ≈ 1.7–1.9), whereas W-15 exhibited K_p values close to unity (AUC_last ≈ 0.9), indicating comparatively lower brain exposure. Together, integrated pharmacological and pharmacokinetic profiling demonstrates that methadone and W-15 differ in receptor engagement, downstream signaling pathways, neurochemical outcomes, and brain exposure, defining distinct drug-specific profiles. [Supported by the NRF (2022R1A6A1A03054419) and the MFDS (23212MFDS218) of Korea.]