Comparison of Prostate-Specific Membrane Antigen Expression Levels in Human Salivary Glands to Non-Human Primates and Rodents

Abstract

Background: Prostate-specific membrane antigen (PSMA) has emerged as a promising target for developing radionuclide therapy (RNT) in prostate cancer; however, accumulation of PSMA-RNT in salivary glands can result in irreversible xerostomia. Methods to prevent PSMA-RNT-related xerostomia could be clinically useful; however, little is known about PSMA expression in salivary glands of preclinical animal models. Using [¹⁸F]DCFPyL autoradiography/biodistribution, PSMA expression levels were determined in salivary glands of various preclinical monkey and rodent species and compared with humans. Methods: Binding affinities (K_d) and PSMA levels (B_max) were determined by in vitro [¹⁸F]DCFPyL autoradiography studies. In vivo rodent tissue uptakes (%ID/g) were determined from [¹⁸F]DCFPyL biodistributions. Results: [¹⁸F]DCFPyL exhibited low nanomolar K_d for submandibular gland (SMG) PSMA across all the species. PSMA levels in human SMG (B_max = 60.91 nM) were approximately two-fold lower compared with baboon SMG but were two- to three-fold higher than SMG PSMA levels of cynomolgus and rhesus. Rodents had the lowest SMG PSMA levels, with the mouse being 10-fold higher than the rat. In vivo rodent biodistribution studies confirmed these results. Conclusions: SMG of monkeys exhibited comparable PSMA expression to human SMG whereas rodents were lower. However, the results suggest that mice are relatively a better small animal preclinical model than rats for PSMA salivary gland studies.

Keywords: PET imaging; PSMA; autoradiography; radionuclide imaging; radionuclide therapy; salivary glands.

FIG. 1.

(A). Structure of [F]DCFPyL. (B). HPLC analysis of [F]DcFPyL (HPLC conditions: Agilent Eclipse plus C18 column (4.6 × 150 mm, 3.5 μm), mobile phase: 10% acetonitrile in 0.1 M ammonium formate, with a flow rate of 1.0 mL/min. Solid line: in-line radio detector; dotted line: UV detector at 254 nm.

FIG. 2.

Representative plots of in vitro autoradiography saturation binding studies of human, non-human primates (baboon, cynomolgus, rhesus) and rodent (mouse, rat) submandibular glands; each point (n = 6 points) is derived from ROI analysis. For each plot: B_t = Bound total; B_nsp = Bound non-specific; B_sp = Bound specific (B_t-B_nsp = B_sp). ROI, regions of interest.

FIG. 3.

(A) Representative autoradiograms showing [F]DCFPyL binding (B_t:[F]DCFPyL) and non-specific binding (B_nsp: [F]DCFPyL+DCFPyL (10⁻⁶M)) to PSMA in the SMGs of various species. (B, C) Binding affinity (K_d, nM) of [F]DCFPyL (B) and PSMA concentrations (C; B_max, nM) in submandibular glands of various species determined from in vitro autoradiograph saturation plots. Each bar represents mean ± SD, n = 3; (D) Representative immunofluorescence staining of PC3-PSMA tumor (i), human submandibular glands (ii), baboon submandibular glands (iii), and mouse submandibular glands (iv) with PSMA antibody (PSMA expression, pink) and DAPI (nuclear stain, blue). SMG, submandibular gland; PSMA, prostate-specific membrane antigen.

FIG. 4.

Comparison of [F]DCFPyL biodistribution in rats and mice at 1 h after injection of [F]DCFPyL. Each bar represents %ID/g ± SD (for mice: %ID/g normalized to 20 g mice; for rats: %ID/g normalized to 125 g rat; n = 5 for each group). SMG, SLG, and PG represent submandibular, sublingual, and parotid glands, respectively.

FIG. 5.

Comparison of Tissue:Blood ratios of [F]DCFPyL in rats and mice at 1 h after injection of [F]DCFPyL. Each bar represents Tissue:Blood ± SD; n = 5 for each group. SMG, SLG, and PG represent submandibular, sublingual, and parotid glands, respectively.

FIG. 6.

Comparison of Tissue:Muscle ratios of [F]DCFPyL in rats and mice at 1 h after injection of [F]DCFPyL. Each bar represents Tissue:Muscle ± SD; n = 5 for each group. SMG, SLG, and PG represent submandibular, sublingual, and parotid glands, respectively.