Anti-PSMA ADC Nanobodies for Therapy and Imaging
While most prostate cancers (PCa) are curable if diagnosed and treated early, few patients will still progress to metastatic PCa (mPCa). The therapeutic landscape for mPCa has changed significantly over the past decade due to the discovery of tumour-associated target antigens that has given rise to the use of radioimmunotherapy (RIT). The most promising target for RIT of PCa to date is prostate-specific membrane antigen (PSMA), a non-secreted type II transmembrane protein produced almost exclusively by prostatic tissue and on tumour-associated neovasculature. Unlike other biomarkers, such as PSA, which may decrease with increasing neoplastic de-differentiation, the level of PSMA has been shown to be upregulated on high-grade, de-differentiated PCa. Monoclonal Abs (mAbs) that bind to the extracellular domain of PSMA were then developed to create a high affinity for mCRPC tissue. mAbs have become indispensable therapeutic and research tools. Given that they are difficult and expensive to produce, they impose a heavy burden on healthcare and research budgets. Moreover, they are not suitable for some applications. First, they are large molecules, which limits their tissue and/or tumor penetration and bio-distribution. Second, they can elicit immune reactions that neutralize their activities, which sometimes limits the long-term use of chimeric and humanized Abs available on the market. Third, mAbs typically have a half-life of several days and this limits their use in molecular imaging because of the intense background signal. The development of antigen-binding fragments (Fab fragments), variable fragments (Fv fragments), and single chain variable fragments (scFv fragments) had stability issues with suboptimal activity vs. conventional Abs because of lower avidity. This lead to the idea of using full yet small antibodies i.e. nanobodies.
Four high affinity (pM), Camel originated nanobodies were developed to bind PSMA. The high affinity (50 pM) clone NB7, was shown to accumulate only in PSMA expressing prostate cancer tumors in vivo, allowing specific targeting of these cells. Moreover, this clone does not inhibit PSMA enzymatic activity and still undergoes internalization. This data suggest that doxorubicin conjugated to PSMA could be delivered specifically into PSMA expressing cells but not any other cell types. We also used non-natural amino acids for site-specific protein conjugation of ADC (doxorubicin as a PoC).
- High affinity nanobodies against an attractive and FDA validated target.
- Technology allows site-specific conjugation of small molecules to proteins such as ADC.
- Nanobodies already confirmed specificity and activity in both imaging and therapy applications.
A provisional application coving the generated antibodies was filed.