After decades in which no novel therapeutic treatments were approved for acute myeloid leukemia (AML), the past few years have seen the approval of several groundbreaking therapies for the disease.1 One of the most rapidly adopted agents is venetoclax (ABT-199), which results in favorable response rates and improved overall survival when used in combination with hypomethylating agents or low-dose cytarabine. Venetoclax exerts its antitumor effects through inhibition of the antiapoptotic protein B-cell lymphoma-2 (Bcl-2), thereby restoring the cellular process of apoptosis.2,3

Unfortunately, as with all current therapies for AML, most patients treated with venetoclax will eventually relapse due to development of resistance mechanisms. For venetoclax, one resistance mechanism that may occur in tumor cells is the upregulation of other antiapoptotic pathway members such as myeloid cell leukemia-1 (Mcl-1) protein, thereby circumventing the inhibitory effect of venetoclax on Bcl-2.4–8 To overcome the development of resistance, rational combinations that target distinct antiapoptotic pathway members and can potentially lead to synergy have become an area of burgeoning translational research.

Combatting resistance to venetoclax

Hematological cancers are radiation sensitive. An example of this phenomenon is highly relevant to AML patients for whom venetoclax treatment has failed. Specifically, radiation-mediated DNA damage is known to result in a decrease in Mcl-1 protein levels.9,10 Delivery of therapeutic levels of total body irradiation via external beam, however, is difficult to implement in hematological malignancies such as AML without significant toxicity.

Alternatively, targeted delivery of radiation can be achieved by antibody radionuclide conjugates (ARCs), where an antibody is linked to a radioactive isotope such as iodine-131 (131I) or actinium-225 (225Ac). ARCs are therefore a potentially more effective and better tolerated alternative to target and deliver high amounts of radiation directly to cancer cells.

There have been several FDA-approved radioimmunoconjugate therapies, including Bexxar, Lutathera, and Azedra, and development of new ARCs is an active area of investigation in the biotechnology and pharmaceutical industries. Actinium Pharmaceuticals, a biotechnology company specializing in ARCs, has performed translational work investigating the use of ARCs to deliver targeted radiation to AML cells that, in addition to delivering potent single-agent tumor-cell-killing activity, may resensitize cells to venetoclax treatment when used in combination with venetoclax (Figure 1).

Figure 1. Proposed mechanism of action of antibody radionuclide conjugate (ARC)
Figure 1. Proposed mechanism of action of antibody radionuclide conjugate (ARC)-mediated resensitization of venetoclax-resistant cells. Leukemia cells exposed to venetoclax undergo apoptosis due to the liberation of Bim from Bcl-2, resulting in the oligomerization of Bax/Bak and initiating the apoptotic cascade. Resistance can develop, however, due to the upregulation of Mcl-1 and subsequent sequestration of Bim. Upon exposure of cells to DNA-damaging treatment such as radiation delivered by an ARC, Mcl-2 levels decrease, and cells are thereby resensitized to venetoclax. (Figure created using Servier Medical Art.)

Preclinical results supporting the ARC–venetoclax combination strategy

CD33 is a validated therapeutic target for AML that is expressed on the majority of leukemic blasts and is not expressed outside of the immune system. Lintuzumab is an anti-CD33 monoclonal antibody that has been clinically evaluated as a naked antibody in multiple clinical trials, although a lack of efficacy led to its discontinuation in clinical trials.11–13 Given the favorable safety profile of lintuzumab, the antibody was pursued as an ARC.

225Ac-labeled radioimmunoconjugate lintuzumab (225Ac-lintuzumab, also known as Actimab-A) is currently being evaluated as a therapeutic for hematologic malignancies including AML and has demonstrated promising results as a single agent.14 Following the completion of single-agent Phase I and Phase II studies, the company has focused clinical development of 225Ac-lintuzumab in mechanism-based clinical combinations. To this end, low doses of 225Ac-lintuzumab are being evaluated in combination with the chemotherapy regimen CLAG-M.

Interim results reported in December 2019 demonstrated an 86% overall response rate with 71% of treated patients confirmed to have minimal residual disease (MRD)-negative status.15 These data lend further support to the idea that a combination of low doses of an ARC with other AML agents may lead to improved outcomes.

As an alpha-emitter, 225Ac emits four alpha particles that each have the potential to cause lethal DNA double-strand breaks. Given the link between DNA damage and decreases in levels of Mcl-1, it was hypothesized that a combination of venetoclax and 225Ac-lintuzumab may reverse resistance in AML cells that do not respond to venetoclax (Figure 1).

To evaluate the effect of a combination of venetoclax and 225Ac-lintuzumab on cell viability in cell lines, AML cells (OCI-AML3) that were known to be resistant to venetoclax were utilized. When IC50 doses of 225Ac-lintuzumab and of venetoclax were tested on the OCI-AML3 cells in culture, the combination of the two agents exhibited a 78% decrease in cell viability, statistically greater than the modest effects of each single agent.16 Western blotting for Mcl-1 in OCI-AML3 cells treated with 225Ac-lintuzumab revealed that levels were markedly decreased following exposure to the ARC, but not when treated with unlabeled lintuzumab.16

Given the promising in vitro data, the combination of 225Ac-lintuzumab and venetoclax was then tested in an animal model of AML using the venetoclax-resistant line OCI-AML3. Venetoclax was administered daily for 21 days, a regimen designed to mimic the clinical therapeutic dosing in AML patients. 225Ac-lintuzumab was given on day 1 alone, or on day 1 followed by the 21 days of venetoclax in the combination treatment group.

Single-agent venetoclax had no impact on tumor growth control, and all mice died by day 28. While 225Ac-lintuzumab as a single agent resulted in a significant control of tumor growth, combination treatment with venetoclax and 225Ac-lintuzumab was superior in terms of survival and tumor growth control and regression. The combination therapy was well tolerated, as indicated by minimal change in body weight in treated mice at the completion of the study.16

Conclusions

In summary, these promising preclinical data support the hypothesis that exposure of venetoclax-resistant AML cells to radiation delivered by a CD33-targeted ARC may resensitize cells to venetoclax and improve outcomes for AML patients. These data underpin the recently initiated clinical trial using 225Ac-lintuzumab in combination with venetoclax in relapsed-refractory AML patients.

The trial is being performed in collaboration with Gary J. Schiller, MD, at the University of California, Los Angeles. Similar to the combination trial of 225Ac-lintuzumab and CLAG-M, the Phase I portion of the trial, a dose escalation of 225Ac-lintuzumab, will be performed to determine the maximum tolerated dose of 225Ac-lintuzumab in combination with venetoclax, followed by a Phase II trial to measure overall response rates.

 

Dale L. Ludwig, PhD, is chief scientific officer and Eileen M. Geoghegan, PhD, is strategic research and business development manager at Actinium Pharmaceuticals.

References
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