Pharmacokinetic/pharmacodynamic correlation and biodistribution of Re (CO)3-phenformin compared with phenformin in murine PDAC model following equimolar intravenous dosing

Re (CO)3-phenformin (Re-phen) is a newly synthesized rhenium biguanide complex recently reported and structurally characterized as a potential anticancer agent. However, a comprehensive evaluation of its in vivo pharmacokinetics (PK), biodistribution (BD), metabolic fate, and PK/pharmacodynamic (PK/PD) properties has not yet been characterized. In this study, we conducted an integrated PK, BD, and PK/PD analysis of Re-phen compared with Phenformin (Phen) following three consecutive equimolar intravenous doses (26.26 µmol/kg per dose) in PDAC-bearing mice, with additional quantification of in vivo biotransformation of Re-phen to Phen to support parent metabolite exposure modeling. Blood collected after the final dose and major organs (tumor, pancreas, liver, kidney, spleen, lung) were analyzed using a rigorously validated LC-MS/MS assay. Re-phen achieved markedly higher tissue deposition than Phen across all organs, including tumor (3.05 ± 1.86 vs 0.05 ± 0.02 nmol/g, **p<0.01), pancreas (61.64 ± 36.24 vs 0.08 ± 0.05 nmol/g, **p<0.01), liver (38.27 ± 20.08 vs 0.16 ± 0.06 nmol/g, ***p<0.001), kidney (22.27 ± 5.80 vs 0.08 ± 0.03 nmol/g, ****p<0.0001), spleen (4.60 ± 1.36 vs 0.29 ± 0.07 nmol/g, ****p<0.0001), and lung (4.51 ± 1.40 vs 0.13 ± 0.04 nmol/g, ****p<0.0001), demonstrating extensive distribution and preferential tissue retention. Re-phen also exhibited measurable in vivo conversion to Phen, with a fraction metabolized (Fm) of ~11% and an AUC Ratio (Phen/Re-phen) of 0.22, indicating that both the parent compound and its converted Phen contribute to systemic exposure. 3D PK/PD analysis demonstrated a statistically significant inverse correlation between the AUCs of Re-Phen and its metabolite Phenformin and terminal tumor weight. By comparison, Phenformin exhibited only a moderate, non-significant 2D correlation with tumor weight (r = -0.800, p = 0.1333), supporting the superior antitumor efficacy of Re-Phen. These results indicate that Re-phen displayed a clear exposure-efficacy relationship and that systemic exposure is a key driver of its anti-tumor response. Collectively, this study provides the first comprehensive in vivo evidence defining the PK, biodistribution, metabolic conversion, and PK/PD behavior of Re-phen, highlighting its superior tumor delivery, measurable parent-metabolite interplay, and strong exposure-driven antitumor effect. These findings support further development of Re-phen as a rhenium-based therapeutic and as a core component of the Re/99mTc-phen theranostic platform for pancreatic cancer.