A DLL3-targeted theranostic for small cell lung cancer: Imaging a low density target with a site-specifically modified radioimmunoconjugate.

Objectives To develop a theranostic tool for the molecular imaging of small cell lung cancer (SCLC).

Methods A pair of novel DLL3-targeted humanized monoclonal antibody constructs were synthesized via two strategies: (1) the non-site-specific conjugation of p-isothiocyanatobenzyl-desferrioxamine (SCN-DFO) with lysine residues on the antibody and (2) the site-specific conjugation of maleimidyl desferrioxamine (Mal-DFO) with engineered cysteine residues within the antibody. These constructs are hereafter referred to as SC16 and SC16-SS, respectively. The immunoreactivity of the constructs was determined via surface plasmon resonance (SPR). Both constructs were radiolabeled with 89Zr to yield radioimmunoconjugates (RIC) with high specific activity (> 8 mCi/mg). Similarly, a humanized anti-hapten antibody was site-specifically labeled with 89Zr to serve as an isotype control. An exploration of the microarray gene expression data from the cancer cell line encyclopedia followed by immunohistochemistry on various SCLC cell lines identified NCI H82 and NCI H69 as having ‘high’ versus ‘low’ expression of DLL3, respectively. Saturation ligand binding assays were performed to evaluate the Bmax of DLL3 expression in H82 versus H69 tumors. PET imaging and acute biodistribution studies were carried out separately at 24 h intervals from 24 through 120 h p.i of the RIC in nude mice bearing sub-cutaneous xenografts of H82 and H69 tumors.

Results Both the SC16 and SC16-SS constructs were > 90% immunoreactive for binding to the DLL3 protein in SPR analyses. PET imaging and biodistrubution studies revealed a higher concentration of radioactivity in H82 tumors than in H69 tumors. This correlated well with the B_max values of 15 nM for H82 tumors versus 6 nM for H69 tumors as derived from saturation binding assays. Both ⁸⁹Zr-SC16 and ⁸⁹Zr-SC16-SS demonstrated excellent specificity for targeting H82 and H69 tumors, which were delineated as early as 24 h p.i. The SC16 RIC yielded a mean tumor uptake of 20 ± 4.4 % ID/g in H82 tumors versus 13 ± 5.5 % ID/g in H69 tumors. ⁸⁹Zr-SC16-SS demonstrated a mean tumor uptake of 24 ± 5.5 % ID/g in H82 tumors versus 13 ± 3.3 % ID/g in H69 tumors. ⁸⁹Zr-SC16-SS also demonstrated higher tumor-to-background ratios than its non-specifically modified cousin. Importantly, the site-specifically radiolabeled isotype control antibody demonstrated only EPR-based uptake in H82 tumors.

Conclusions The site-specifically labeled SC16-SS demonstrates the potential to serve as a theranostic to facilitate the non-invasive delineation of SCLC and the stratification of patients for treatment with its antibody drug conjugate avatar that has shown promising therapeutic results in ongoing Phase I clinical trials. This study also demonstrates the feasibility of using low-density tumor biomarkers such as DLL3 for the molecular imaging of cancer and illustrates the pharmacokinetic benefit of using site-specifically modified immunoconjugates over their non-specifically modified cousins. Figure 1. 120 h p.i. PET images of SC16 and SC16.56 radioimmunoconjugates in sub-cutaneous SCLC xenograft models: Upper panels show coronal slices, and lower panels show the corresponding maximum intensity projections of - A) H82 (high DLL3 expression) tumor-bearing mouse injected with the non-specifically labeled ⁸⁹Zr-SC16 (SC16); B) H69 (low DLL3 expression) tumor-bearing mouse injected with SC16; C) H82 (high DLL3 expression) tumor-bearing mouse injected with the site-specifically labeled ⁸⁹Zr-SC16 (SC16-SS); and D) H69 (low DLL3 expression) tumor-bearing mouse injected with SC16-SS. All coronal images are scaled to 30 % ID/g.

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