Revisiting Cinacalcet HCl Hepatic Disposition: Insights from Isolated Perfused Rat Liver

Hepatic drug handling determines systemic exposure, yet the balance between tissue uptake, intracellular binding, and metabolic clearance is difficult to dissect in vivo. Cinacalcet hydrochloride (CINA), a calcimimetic used to manage secondary hyperparathyroidism, has low oral bioavailability and is widely described as a high-extraction drug subject to extensive hepatic metabolism. Whether tissue sequestration rather than biotransformation underlies its rapid hepatic removal has not been tested directly in an intact ex vivo liver model.

We investigated the hepatic clearance of cinacalcet hydrochloride (CINA) using the isolated perfused rat liver (IPRL) model. A validated high-performance liquid chromatography (HPLC) method was established to quantify CINA and its primary metabolite, M7, in both liver tissue and perfusate samples. To evaluate the influence of perfusate composition and protein binding, experiments were conducted under single-pass perfusion modes, in the presence and absence of bovine serum albumin (BSA).

Dose recovery and mass balance analysis indicated that ~93% of the administered dose was recovered: ~63% in liver tissue and ~29% in the outlet perfusate. No metabolites were detected in any sample, pointing to tissue uptake and retention as the dominant clearance mechanism. Drug loss to plastic perfusion tubing was substantial and was only partially offset by BSA. Inclusion of BSA also shifted the apparent drug distribution between liver and perfusate.

In the IPRL, hepatic tissue sequestration rather than metabolic conversion was the dominant clearance mechanism for CINA. Non-specific adsorption to plastic perfusion tubing and the protein content of the perfusion medium were important confounders that must be controlled in future ex vivo studies.

CINA was extensively retained in rat liver tissue with negligible in situ metabolism over 90 min of single-pass perfusion, raising the question of whether metabolic extraction truly explains its low oral bioavailability in vivo.