Targeting ROR1 Using the Antibody Drug Conjugate Vls-101 in Aggressive Mantle Cell Lymphoma

ROR1 is a transmembrane receptor with tightly controlled expression during embryonic development. While it is expressed on multiple tumor types it is not expressed in normal adult tissues. ROR1-positive B cell non-Hodgkin's lymphomas (B-NHL) include mantle cell lymphoma (MCL), and diffuse large B cell lymphoma. Given its unique expression pattern, ROR1 represents a tumor-specific therapeutic target with little or no normal tissue toxicity. VLS-101, utilizes the UC-961 anti-ROR1 antibody which is conjugated to monomethyl auristatin E (MMAE) via a cleavable linker. VLS-101/ROR1 complex induce internalization facilitates the release of a cytotoxic agent resulting in killing of ROR1-expressing cancer cells. With the rise of novel targeted therapies, resistance has been a major challenge among MCL patients. MCL is one of the most therapeutically resistant and aggressive forms of NHL-B, making it a challenging cancer to treat. The advancement of ibrutinib, a Bruton's tyrosine kinase (BTK) inhibitor, has been a beneficial change; however, ibrutinib-resistant MCL remains an urgent unmet clinical need. Furthermore, MCLs are either intrinsically resistant to ibrutinib, as well as to venetoclax (Bcl-2 inhibitor) and CAR T-cell therapy, or they may acquire resistance after a short response to these therapies. While many studies have looked into overcoming each of these different types of resistance, no studies have been conducted to overcome a multiple-resistance phenotype simultaneously. Therefore, identifying effective therapies that target these resistance mechanisms is crucial. In this study, we assessed the efficacy of VLS-101 in preclinical models of MCL with intrinsic or aquired resistance to ibrutinib, venetoclax, and CD19 CAR T therapies. By using MCL cell lines, primary patient samples and our established MCL patient-derived xenografts, we explored the expression of ROR1 in MCL and investigated the effects of in vitro and in vivo effects ROR1 targeting using VLS-101.

To assess total and cell surface ROR1 expression we used flow cytometry (FACS), western blot analysis, and reverse-transcriptase-polymerase chain reaction (RT-PCR). Cell viability and apoptosis assays were performed to evaluate the in vitro efficacy of VLS-101 in MCL cell lines and primary patient MCL samples. To assess the apoptosis and cell cycle arrest in cells treated with VLS-101, we used annexin V/propidium iodide-staining followed by flow cytometry analyses. In vivo efficacy was tested using various PDX models with various multiple resistances.

All 5 cell lines and all 9 PDX models of MCL tested showed high ROR1 mRNA and protein expression levels, whereas 3 out of 4 primary human samples expressed cell surface ROR1. VLS-101 treatment showed anti-MCL activity at the concentrations of 0.3-9 μg/ml in most of cell models tested, which correlated with a significant G2/M cell cycle arrest. Furthermore, VLS-101 induced a time- and dose-dependent apoptosis, as shown by increases in annexin V/propidium iodide-staining.

VLS-101 treatment of ibrutinib-venetoclax dual-resistant ROR1⁺ PDX model resulted in significant regressions of the tumor bearing spleens and livers when compared to vehicle controls (p=0.0001 and p=0.002 respectively). In the ibrutinib-CD19 CAR T dual-resistant ROR1⁺ model, vehicle-treated animals showed s.c. tumor masses with a mean volume of 1440 mm³, whereas tumor masses were barely palpable (mean = 160 mm³) or had completely regressed in animals treated with VLS-101 at 1.0 mg/kg or 2.0 mg/kg, respectively. VLS-101 showed no adverse effects as monitored by animal clinical observations and weekly body weight measurements.

Collectively, these results indicate that ROR1 expression on MCL cells can be utilized for selective targeting. These preclinical data document that the MMAE-containing ADC, VLS-101, can cause cell cycle arrest and induce apoptosis in vitro and safely induce tumor regressions in highly resistant in vivo PDX models of MCL derived from patient tumors. Importantly, our study revealed that even the heavyly pretreated tumors in the clinical setting may express targetable levels of ROR1 and that targeting ROR1 using ADC is a promising approach for the treatment of MCL. Building on these types of results, a Phase 1 clinical trial on VLS-101 (NCT03833180) is ongoing in patients with lymphoid cancers.