Radiosynthesis and Development of Peptide Decorated Dendrimers as a Drug Delivery Platform for Oncological Application.

Introduction: One of the most validated targets in prostate cancer is prostate-specific membrane antigen (PSMA), a membrane anchored protease highly upregulated in patients with prostate cancer. While a majority of all prostate gland adenocarcinomas overexpress PSMA, recent literature provides overwhelming data showing increased PSMA expression in different target organs, including the kidney, proximal small intestine and salivary glands. High-affinity ligands for PSMA inhibition have been developed and range from small-molecule inhibitors; 2-(phosphonomethyl)pentanedioic acid (PMPA) to a low molecular weight peptide consisting of a GUL (glutamate-urea-lysine) or GUG (glutamate-urea-glutamate) motif, as seen in all three ([⁶⁸Ga]PSMA-11, [¹⁸F]DCFPyL, [¹⁷⁷Lu]PSMA-617) FDA approved radiopharmaceuticals for prostate cancer. However, despite their success, increased PSMA expression across kidney tissues directly impact renal tracer uptake, which, can be dose-limiting. This is followed by subsequent impairment or decline in kidney function, which impacts drug clearance and the intended therapeutic outcome. To improve the background-to-tumor ratio a new approach to radioligand design is necessary. This study aims to exploit the physiochemical properties of Bis-MPA based dendrimers to control kidney filtration and uptake via polymer size manipulation and structural modifications to produce robust pharmacokinetic profiles compared to pre-existing probes.

Methods: In this study we compare the targeting efficiency of peptide decorated dendrimers (PDDs) to previously radiolabeled PSMA scaffolds containing a GUL motif using a series of Bis-MPA derivatives. Generation 3, 4 and 5 dendrimers were synthesized via divergent synthesis and conjugated with prepared peptide derivatives on the periphery. The desired product was purified using size exclusion chromatography and provided in good yield by the Adronov Group as suspended crystals for future radiosynthetic work. An iodonium salt was prepared as a precursor to synthesize [¹²⁵I]-1-(azidomethyl)-3-iodobenzene to ensure a facile click reaction between the azide and its designated click partner. Characterization and purification of the [¹²⁵I]-GUL-Dendrimer will be achieved using RP-HPLC, followed by a TLC scanner to the track purity of the isolated compound. The radiolabeled PDD will be used in subsequent In-vitro assays and compared to its unlabeled partner to verify radioiodination does not alter therapeutic efficacy. Pharmacokinetic evaluations will be done using models previously established by the McMaster Radiochemistry Group followed by biodistribution studies in tumor models.

Results: The iodonium salt was synthesized in 73% yield. Click reactions were tested (prior to preparing the final labeled click partners) with benzyl azide and unlabeled bis-MPA dendrons to provide a yield of 85%.

Conclusions: This work aims to develop a versatile dendrimer-based drug delivery platform for targeted drug delivery using PSMA as a starting model for future disease applications to circumvent pre-existing pharmacokinetic issues in PSMA radiopharmaceutical development. Future directions include the use of bifunctional chelators to coordinate a variety of radiometals for applications in imaging studies and clinical translation upon any promising data obtained. The successful development and evaluation of our prepared probes will pave a way for next generation PSMA therapeutics and guide the design of dendrimer based therapeutic platforms for the ever-growing disease state of prostate cancer.