Validity of Anti-PSMA ScFvD2B as a Theranostic Tool: A Narrative-Focused Review

Abstract

Prostate cancer (PCa) is the second leading cause of cancer among men, and its diagnosis and adequate staging are fundamental. Among the biomarkers identified in recent years for PCa management, prostate-specific-membrane-antigen (PSMA), physiologically expressed at a low level on healthy prostate and in other normal tissues and highly overexpressed in PCa, represents a reliable marker ideal for imaging and therapy. The development of anti-PSMA antibodies, such as D2B, demonstrated slow clearance of intact antibodies compared with fragments resulting in low tumor-to-blood ratios; however, the modular structural and functional nature of antibodies allowed the generation of smaller fragments, such as scFvs. In this review of the anti-PSMA antibody fragment scFvD2B, we combined further characterization of its biomolecular and tissue cross-reactivity characteristics with a comprehensive summary of what has already been performed in preclinical models to evaluate imaging and therapeutic activities. A molecular dynamics study was performed, and ScFvD2B occupied a limited conformational space, characterized by low-energy conformational basins, confirming the high stability of the protein structure. In the cross-reactivity study, the weak/absent immunoreactivity in non-tumor tissues was comparable to the PSMA expression reported in the literature. Biodistribution studies and therapeutic treatments were conducted in different animal models obtained by subcutaneous or locoregional injection of PSMA-positive-versus-negative xenografts. The maximum tumor uptake was observed for 123I(SPECT), 124I(PET), and optical imaging, which avoids kidney accumulation (compared with radiometals) and leads to an optimal tumor-to-kidney and tumor-to-background ratios. Regarding its possible use in therapy, experimental data suggested a strong and specific antitumor activity, in vitro and in vivo, obtained using CAR-T or NK-92/CAR cells expressing scFvD2B. Based on presented/reviewed data, we consider that scFvD2B, due to its versatility and robustness, seems to: (i) overcome some problems observed in other studied scFvs, very often relatively unstable and prone to form aggregates; (ii) have sufficient tumor-to-background ratios for targeting and imaging PSMA-expressing cancer; (iii) significantly redirect immune killing cells to PSMA-positive tumors when inserted in second-generation CAR-T or NK-92/CAR cells. These data suggest that our product can be considered the right reagent to fill the gap that still exists in PCa diagnosis and treatment.

Theranostic 64Cu-DOTHA2-PSMA allows low toxicity radioligand therapy in mice prostate cancer model

“…However, the modular nature of antibodies allowed the generation of smaller fragments, such as scFvs that can overcome some problems observed with antitbodies. Indeed, it has been shown that scFvs can offer appropriate tumor-to-background ratios for imaging and treating PSMA-expressing cancer ( 12 ).…”

Development and Characterization of 99mTc-scFvD2B as a Potential Radiopharmaceutical for SPECT Imaging of Prostate Cancer

“…The tumor uptake of 111 In-labeled IgG-D2B was shown to be higher than that of the commercial ProstaScint ® but suffered from the same molecular size limitations [ 28 ]. Therefore, to circumvent this problem, a single-chain variable fragment of the IgG-D2B antibody (termed scFvD2B) was generated [ 29 ]. It has been reported that small antibody fragments can access binding sites more uniformly and penetrate solid tumors more homogeneously and efficiently than full-length antibodies [ 30 ].…”

“…In vivo imaging of scFvD2B radiolabeled with different radionuclides ( 123 I, 111 In and 177 Lu) confirmed the relatively fast and high antigen-positive tumor uptake (from 3 h post-injection), with a short circulatory half-life and rapid clearance from most of the non-target tissues, allowing labeling with radionuclides of relatively short physical half-life (t 1/2 ) such as 99m Tc (t 1/2 = 6 h) [ 31 , 32 , 33 ]. Moreover, due to its larger size compared to PSMA-derived peptides, scFvD2B can be conjugated to different chelating agents without drastically affecting its binding affinity and pharmacokinetic properties [ 29 , 32 ].…”

Antibody Based Delivery of Toxins and Other Active Molecules for Cancer Therapy

“…The validity of the anti-PSMA antibody fragment scFvD2B as a theranostic tool was exploited in a review article combining the characterization of its biomolecular and tissue cross-reactivity characteristics with a comprehensive summary of what has already been performed in preclinical models, to evaluate imaging and therapeutic activities. The authors concluded that scFvD2B being a versatile and robust molecule, with sufficient tumor-to-background ratios for targeting and imaging PSMA-expressing cancer, can be considered a good potential reagent for PCa diagnosis and treatment [ 13 ].…”