Preclinical assessment of ⁶⁸Ga‐PSMA‐617 entrapped in a microemulsion delivery system for applications in prostate cancer PET/CT imaging

Abstract

It has in recent years been reported that microemulsion (ME) delivery systems provide an opportunity to improve the efficacy of a therapeutic agent whilst minimising side effects and also offer the advantage of favourable treatment regimens. The prostate-specific membrane antigen (PSMA) targeting agents PSMA-11 and PSMA-617, which accumulate in prostate tumours, allow for [ 68 Ga]Ga 3+ -radiolabelling and positron emission tomography/computed tomography (PET) imaging of PSMA expression in vivo. We herein report the formulation of [ 68 Ga]Ga-PSMA-617 into a ME ≤40 nm including its evaluation for improved cellular toxicity and in vivo biodistribution. The [ 68 Ga]Ga-PSMA-617-ME was tested in vitro for its cytotoxicity to HEK293 and PC3 cells. [ 68 Ga] Ga-PSMA-617-ME was administered intravenously in BALB/c mice followed by microPET/computed tomography (CT) imaging and ex vivo biodistribution determination. [ 68 Ga]Ga-PSMA-617-ME indicated negligible cellular toxicity at different concentrations. A statistically higher tolerance towards the [ 68 Ga]Ga-PSMA-617-ME occurred at 0.125 mg/mL by HEK293 cells compared with PC3 cells. The biodistribution in wild-type BALB/C mice showed the highest amounts of radioactivity (%ID/g) presented in the kidneys (31%) followed by the small intestine (10%) and stomach (9%); the lowest uptake was seen in the brain (0.5%). The incorporation of [ 68 Ga]Ga-PSMA-617 into ME was successfully demonstrated and resulted in a stable nontoxic formulation as evaluated by in vitro and in vivo means.

Preclinical Assessment Addressing Intravenous Administration of a [68Ga]Ga-PSMA-617 Microemulsion: Acute In Vivo Toxicity, Tolerability, PET Imaging, and Biodistribution

“…Further optimization of the purification process post radiolabeling provided robust [ 68 Ga]Ga-PSMA-617 yields (LE > 95%, RCP: >98%; n = 5). The radio-HPLC based product identification (retention time 6.27 ± 0.51 min) was more consistent to previously published results [ 11 ]. Low evidence of colloidal- 68 Ga was observed (>5.0%, ITLC-based).…”

“…First, measurement of ME (blank) and 68 Zn-PSMA-617-ME samples for droplet size, distribution (PDI) and surface charge (Zeta potential) demonstrated the following differences: (I) The average particle size of the microemulsion (72.23 ± 0.18 nm) was significantly higher compared to 68 Zn-PSMA-617-ME (27.61 ± 1.11 nm; p < 0.01), which implied that the addition of saline increased the droplet particle size whereas the addition of 68 Zn-PSMA-617-ME maintained a narrow droplet size (average particle size ME lacking saline ranged between 20 and 40 nm with a distribution between 0.10 and 0.30 [ 11 ]); (II) the PDI for the 68 Zn-PSMA-617-ME (0.53 ± 0.01) was higher than that of the ME (0.26 ± 0.01, p < 0.001) which may have been a result of the addition of 68 Zn-PSMA-617 leading to the formation of a system with multiple particle sizes; and (III) neutral conditions (pH ~7.0) caused no differences in the negative zeta potential between ME and 68 Zn-PSMA-617-ME (both <−5.0 mV). Second, Table 1 summarizes the results comparing the characterization of 68 Zn-PSMA-617 samples with 68 Zn-PSMA-617-ME formulations (n = 3).…”

“…The animals' blood samples were drawn and stored in BD Vacutainer, Fisher Scientific Inc. (Schwerte, Germany). 68 Ga]Ga-PSMA-617 and [ 68 Ga]Ga-PSMA-617-ME The principle of the radiosynthesis for [ 68 Ga]Ga -PSMA-617 and formulation into the water-in-oil-in-water (w/o/w) microemulsion was adopted from a previously described method [11]. Briefly, the generator-derived 68 Ga-radioactivity was eluted by performing manual eluate fractionation as described by Breeman et al, 2005 [23] and measured in a dose calibrator (CRC15, Capintec Inc, Pittsburgh, PA, USA) using 0.6 M hydrochloric acid Sigma Aldrich (St Louis, MO, USA).…”

“…Characterization of ME and 68 Zn-PSMA-617-ME Size distribution, droplet size measured by dynamic laser scattering (DLS), and zeta potential measured via Laser Doppler velocimetry were all performed using a Malvern Zetasizer Nano ZS (Malvern Instruments, Worcestershire, United Kingdom) using protocols from previously reported methods [11]. The pH and conductivity values of each microemulsion were measured at ambient temperature using a PC 8, Accsen pH and conductivity meter (Lasec, Midrand, South Africa).…”

“…The application of 177 Lu-based targeted radioligand therapy (TRT) is currently the most reported form; i.e., [ 68 Ga]Ga-PSMA would be a considerable option to diagnose prostate cancer followed by the treatment therapy option of [ 177 Lu]Lu-PSMA-617-TRT. Success with TRT is based on the fact that PSMA expression increases in metastatic and hormone refractory prostate cancer, which makes it an ideal target for positron emission tomography/computed tomography (PET/CT) imaging and therapy [11].…”