Researchers in the lab

Credit: Rhoda Baer

PHILADELPHIA—A pair of engineered toxin bodies (ETBs) can successfully treat Burkitt lymphoma and multiple myeloma, according to preclinical research presented at the AACR conference Hematologic Malignancies: Translating Discoveries to Novel Therapies.

The ETBs, known as MT-4007 and MT-4007-D, work by targeting CD38.

They greatly reduced tumor burden and improved survival in mouse models. And they were well-tolerated, even at the highest doses administered.

“In this study, we found that the growth of human cancer cells in mice was substantially decreased, or the cells were even eliminated, following treatment with our investigational CD38-targeted therapy,” said Erin K. Willert, PhD, of Molecular Templates Inc., in Georgetown, Texas.

Dr Willert and her colleagues explained that ETBs are derived from the ribosome-inactivating alpha subunit of Shiga-like toxin 1 (SLT-1A). They have been engineered to contain a target binding domain fused to a modified SLT-1A protein, which allows for delivery to a cell surface target—in this case, CD38.

Upon binding to a CD38-expressing cell, the ETB enters the cell, routes to the cytosol, halts protein synthesis, and kills the cell.

The researchers first tested MT-4007 and MT-4007-D in a range of human cell lines. The agents exhibited cytotoxicity in CD38+ Burkitt lymphoma and multiple myeloma cell lines (H929, Daudi, ST486, and Raji). But neither agent proved cytotoxic in CD38- cell lines (U266, SKBR3, and HCC1954).

The team then moved on to test MT-4007 in a mouse model of Burkitt lymphoma. Following injection with Daudi-Luc cells, mice received no treatment or MT-4007 at 0.05 mg/kg, 0.5 mg/kg, or 2 mg/kg on days 3, 5, 8, 10, and 12.

Treated mice exhibited significantly reduced tumor burden compared to controls. The mean tumor burden for mice that received MT-4007 at 0.05 mg/kg was 29% of the control tumor burden (P<0.0001). It was 0.4% for mice that received 0.50 mg/kg (P<0.0001) and 0.02% for mice that received 2 mg/kg (P<0.0001).

In a model of multiple myeloma, MT-4007-D provided a dose-dependent delay in tumor growth. After receiving injections of H929 cells, mice received no treatment or MT-4007-D at 0.5 mg/kg, 2 mg/kg, or 3 mg/kg on days 1, 3, 5, 8, 10, and 12.

The researchers assessed efficacy by measuring the time to endpoint, which was a tumor volume of 2000 mm³. The median time to endpoint was 22.3 days in controls, 21.2 days in the 0.5 mg/kg arm (not significant), 24.5 days in the 2 mg/kg arm (P=0.004), and 26.2 days in the 3 mg/kg arm (P=0.04).

The team assessed safety using body weight. They found that all treated groups of mice maintained a stable weight, suggesting MT-4007-D is well-tolerated.

The researchers also noted that, in a previous dose-finding study, the maximum-tolerated dose of MT-4007 was not reached at the highest dose administered to mice (2 mg/kg), which suggests MT-4007 is well tolerated as well.

Finally, Dr Willert and her colleagues found that MT-4007 extends survival in models of Burkitt lymphoma. The team euthanized mice if they had a greater than 20% loss in body weight or symptoms such as hind limb paralysis.

In the control group, all 10 mice died, and the median survival was 34 days. In the 0.5 mg/kg treatment group, 5 mice died, and the median survival was 59.5 days (P=0.0002).

One mouse died in the 0.5 mg/kg group (P<0.0001), and none of the mice died in the 2 mg/kg group (P<0.0001). The median survival was undefined for both groups.

Dr Willert said these results suggest the ETBs should be moved forward to clinical trials in CD38+ B-cell malignancies such as multiple myeloma. And because the ETBs work differently from other treatments, they might prove effective in relapsed or refractory patients.

However, more preclinical research is needed before the ETBs can be tested in patients. MT-4007-D is under investigation in preclinical studies now.

This research was funded by Molecular Templates Inc., makers of MT-4007 and MT-4007-D.

ETBs prove effective against lymphoma and myeloma