Vor Bio's dual stem cell, ADC treatment shows consistent engraftment with reduced toxicity, but response data absent

New data from Vor Bio show that its hematopoietic stem cells have, so far, successfully engrafted in the first seven patients with relapsed or refractory acute myeloid leukemia and that the new cells have been shielded from toxicity associated with antibody-drug conjugates (ADCs). 

The results, released Thursday, are an early sign that the gene-edited transfusion process can limit side effects from the two-pronged regimen, and cancer cells, rather than healthy ones, are squarely in the crosshairs. Three patients have received a maintenance dose of Pfizer's Mylotarg, an approved ADC, which constitutes the second stage of the treatment course. Vor reports that those patients have “experienced hematologic protection,” suggesting that the Mylotarg dose can be increased without toxicity issues and that the company could treat engrafted patients with a CD33-targeting, allogeneic CAR-T without issue. The data are set to be presented Friday at the annual International Conference on Relapse After Transplant and Cellular Therapy in Los Angeles.

Vor did not include response rate data in its update. However, CEO Robert Ang is clear-eyed about the potential benefits in the hard-to-tackle patient population. He conceded that most of the patients in the trial will relapse, given the dire state of the disease, which in turn puts a lower emphasis on response rate data. 

“Mylotarg is not going to be 100% effective here,” he said. So far, three of the engrafted patients have relapsed, including one who received Mylotarg. More important for Ang and Vor was to prove their central hypothesis that editing out CD33 from stem cells’ surface target will protect them after transfusion. 

If and when patients in the trial relapse, they’ll have the chance to add on either a 0.5-mg induction course of Mylotarg—more frequent than the maintenance dose—or an allogenic version of Vor’s CAR-T therapy, VCAR33. Heading into 2024, Ang aims to accelerate recruitment to gain a more comprehensive look at the efficacy of post-relapse treatment.

“That's data we want to see, that's data that investors want to see, and so there's really strong alignment here,” he said.