Lead-212 Radio-DARPin Therapeutic (RDT) targeting delta-like ligand 3 (DLL3) shows promising preclinical antitumor efficacy and tolerability in small cell lung cancer (SCLC)

Introduction: Radioligand therapy (RLT) has shown strong clinical potential for the treatment of certain neuroendocrine and prostate tumors but is currently limited in absence of suitable targeting vectors in other cancer types. DARPins (Designed Ankyrin Repeat Proteins) are a versatile class of small binding proteins that can be generated against a broad range of tumor targets. They have several intrinsic properties that make them attractive vectors for targeted radiotherapy: (i) a short systemic half-life combined with very high affinity, resulting in efficient tumor uptake, deep penetration and long retention; (ii) a fast systemic clearance and high selectivity, leading to low exposure in blood and healthy tissues; (iii) a robust architecture and high thermal stability to potentially tolerate harsh radiolabeling conditions for certain isotopes and enable engineering approaches not compatible with other protein scaffolds. Nevertheless, use of small-size, protein-based targeting agents like DARPins in radiotherapy is often limited by toxicity related to strong kidney retention and typical renal clearance of small proteins. Previously, we showed that by optimizing the surface of the DARPin scaffold through extensive engineering it is feasible to overcome this limitation. Building on the optimized properties of our RDT platform, including low kidney accumulation, we selected DLL3 as target for our first RDT candidate for combination with Lead-212 (²¹²Pb), which is an alpha particle-emitting isotope that has a half-life of 10.6 h and is well-suited for targeted alpha therapy due to its unique physical properties. Targeted alpha therapy is an emerging RLT that selectively delivers alpha-particle radiation to cancer cells while minimizing systemic toxic effects. DLL3 is highly upregulated in SCLC, an aggressive lung cancer type with limited treatment options. Recent advances in clinical development of DLL3-targeted T cell-engagers confirmed DLL3 as an attractive target for treatment of SCLC

Methods: Building on the absence of cysteines in the DARPin scaffold, we generated single-cysteine versions for site-specific conjugation to different linker/chelators using maleimide chemistry. Picomolar affinity binders against DLL3 were selected from DARPin libraries by ribosome display, then conjugated to the DOTAM chelator and radiolabelled with ²¹²Pb for in-vivo biodistribution studies. To reduce kidney retention, several surface optimized variants were engineered. Candidates with low kidney retention were further optimized by affinity maturation and/or combined with various half-live extension (HLE) approaches. Best variants were used for safety and therapeutic assessment.

Results: In vitro characterization showed that engineered DLL3 DARPins maintained good biophysical properties and high affinities to the target antigen. In vivo biodistribution studies with surface engineered ²¹²Pb-DARPins showed strongly improved profiles vs parental binders. Kidney accumulation was reduced at 4 h post injection, while tumor uptake was not affected. This effect was confirmed with different DLL3 DARPin families, suggesting a range of applicable approaches. HLE approaches achieved a clear increase in tumor uptake and retention. The different strategies resulted in an additive effect on the ²¹²Pb-DLL3-RDT candidates, improving the tumor-to-kidney ratio from 1:26 to 1:1.5 in NCI-H82 cells and even above 1 in MC38-hDLL3 mouse models. Strong tumor reduction was confirmed in a mouse efficacy study, while no major toxicity was observed in a dose-finding study up to 40 μCi of injected ²¹²Pb-DLL3 molecules.

Conclusions: These preclinical results support ²¹²Pb-Radio-DARPin therapeutics against DLL3 to be developed as a promising treatment option for SCLC with encouraging in vivo antitumor activity and a good safety profile for our first DLL3-RDT candidate. The data further highlights the versatility and efficiency of the RDT platform for the development of next-generation RLT product candidates.