Bioequivalence assessment of high-capacity polymeric micelle nanoformulation of paclitaxel and Abraxane® in rodent and non-human primate models using a stable isotope tracer assay.

The in vivo fate of nanoformulated drugs is governed by the physicochemical properties of the drug and the functionality of nanocarriers. Nanoformulations such as polymeric micelles, which physically encapsulate poorly soluble drugs, release their payload into the bloodstream during systemic circulation. This results in three distinct fractions of the drug-nanomedicine: encapsulated, protein-bound, and free drug. Having a thorough understanding of the pharmacokinetic (PK) profiles of each fraction is essential to elucidate mechanisms of nanomedicine-driven changes in drug exposure and PK/PD relationships pharmacodynamic activity. Here, we present a comprehensive preclinical assessment of the poly (2-oxazoline)-based polymeric micelle of paclitaxel (PTX) (POXOL hl-PM), including bioequivalence comparison to the clinically approved paclitaxel nanomedicine, Abraxane®. Physicochemical characterization and toxicity analysis of POXOL hl-PM was conducted using standardized protocols by the Nanotechnology Characterization Laboratory (NCL). The bioequivalence of POXOL hl-PM to Abraxane® was evaluated in rats and rhesus macaques using the NCL's established stable isotope tracer ultrafiltration assay (SITUA) to delineate the plasma PK of each PTX fraction. The SITUA study revealed that POXOL hl-PM and Abraxane® had comparable PK profiles not only for total PTX but also for the distinct drug fractions, suggesting bioequivalence in given animal models. The comprehensive preclinical evaluation of POXOL hl-PM in this study showcases a series of widely applicable standardized studies by NCL for assessing nanoformulations prior to clinical investigation.

Application of a Scavenger Receptor A1-Targeted Polymeric Prodrug Platform for Lymphatic Drug Delivery in HIV.

We have developed a macromolecular prodrug platform based on poly(l-lysine succinylated) (PLS) that targets scavenger receptor A1 (SR-A1), a receptor expressed by myeloid and endothelial cells. We demonstrate the selective uptake of PLS by murine macrophage, RAW 264.7 cells, which was eliminated upon cotreatment with the SR-A inhibitor polyinosinic acid (poly I). Further, we observed no uptake of PLS in an SR-A1-deficient RAW 264.7 cell line, even after 24 h incubation. In mice, PLS distributed to lymphatic organs following i.v. injection, as observed by ex vivo fluorescent imaging, and accumulated in lymph nodes following both i.v. and i.d. administrations, based on immunohistochemical analysis with high-resolution microscopy. As a proof-of-concept, the HIV antiviral emtricitabine (FTC) was conjugated to the polymer's succinyl groups via ester bonds, with a drug loading of 14.2% (wt/wt). The prodrug (PLS-FTC) demonstrated controlled release properties in vitro with a release half-life of 15 h in human plasma and 29 h in esterase-inhibited plasma, indicating that drug release occurs through both enzymatic and nonenzymatic mechanisms. Upon incubation of PLS-FTC with human peripheral blood mononuclear cells (PBMCs), the released drug was converted to the active metabolite FTC triphosphate. In a pharmacokinetic study in rats, the prodrug achieved ∼7-19-fold higher concentrations in lymphatic tissues compared to those in FTC control, supporting lymphatic-targeted drug delivery. We believe that the SR-A1-targeted macromolecular PLS prodrug platform has extraordinary potential for the treatment of infectious diseases.

Distinguishing Pharmacokinetics of Marketed Nanomedicine Formulations Using a Stable Isotope Tracer Assay.

The pharmacokinetics of nanomedicines are complicated by the unique dispositional characteristics of the drug carrier. Most simplistically, the carrier could be a solubilizing platform that allows administration of a hydrophobic drug. Alternatively, the carrier could be stable and release the drug in a controlled manner, allowing for distribution of the carrier to influence distribution of the encapsulated drug. A third potential dispositional mechanism is carriers that are not stably complexed to the drug, but rather bind the drug in a dynamic equilibrium, similar to the binding of unbound drug to protein; since the nanocarrier has distributional and binding characteristics unlike plasma proteins, the equilibrium binding of drug to a nanocarrier can affect pharmacokinetics in unexpected ways, diverging from classical protein binding paradigms. The recently developed stable isotope tracer ultrafiltration assay (SITUA) for nanomedicine fractionation is uniquely suited for distinguishing and comparing these carrier/drug interactions. Here we present the the encapsulated, unencapsulated, and unbound drug fraction pharmacokinetic profiles in rats for marketed nanomedicines, representing examples of controlled release (doxorubicin liposomes, Doxil; and doxorubicin HCl liposome generic), equilibrium binding (paclitaxel cremophor micelle solution, Taxol generic), and solubilizing (paclitaxel albumin nanoparticle, Abraxane; and paclitaxel polylactic acid micelle, Genexol-PM) nanomedicine formulations. The utility of the SITUA method in differentiating these unique pharmacokinetic profiles and its potential for use in establishing generic nanomedicine bioequivalence are discussed.