Antibody-drug conjugates (ADCs) have been shown to be a valid option for treatment in acute myeloid leukemia (AML). At present, the majority of ADCs available target CD33 (a 67-kDa type I transmembrane receptor), based on its broad expression on AML blasts. However, its expression on normal hematopoietic stem cells (HSCs) indicates that prolonged cytopenias among multiple hematopoietic lineages may limit the clinical benefit of CD33 targeting agents. In a study published in the June 2018 issue of Clinical Cancer Research, Bing Zheng, Shang-Fan Yu and colleagues aimed to develop a next-generation ADC targeting C-type lectin-like molecule-1 (CLL-1) for the treatment of AML.
To do this, they first validated CLL-1, a leukemic stem cell marker as a target for AML treatment as compared to CD33. Surface expression of CLL-1 and CD33 was analyzed on AML blasts from samples obtained from 70 patients with AML using flow cytometry. CLL-1 exhibited similar frequency as compared to CD33 on AML blasts. Further analysis of CLL-1 on bone marrow samples from patients with AML demonstrated its expression on CD34 AML blasts but not on lymphocytes or platelets. Interestingly, CLL-1 expression was not detected on CD34+/CD38- stem cells obtained from healthy donors. According to the researchers, this finding indicates that “CLL-1 shares similar prevalence and trafficking properties that make CD33 an excellent ADC target for AML, but lacks the HSC expression that may be hampering current CD33 and CD123 targeted ADCs.”
The hematopoietic potential of CLL-1 was evaluated next. To identify the stage at which CLL-1 expression begins during hematopoiesis, the investigators used a panel of markers to define each differential stage of CD34+ normal hematopoietic stem cells and progenitor cells (HSPCs). CLL-1 was expressed on at least half of the population of CD34+/CD38+/CD10-/CD45RA-/CD135+ common myeloid progenitor (CMP) cells and increased levels on CD34+/CD38+/CD10-/CD45RA+/CD135+ granulocyte-monocyte progenitor (GMP) cells. CLL-1 expression was lost on the next differentiated cells, megakaryocyte-erythroid progenitor, thus indicating that CLL-1 is an “early myeloid lineage marker that resides during the differentiation from CMP to GMP cells”. Using colony forming assay, it was shown that depletion of CD34+/CLL1+ progenitor cells does not impair the normal hematopoietic potential or induce persistent thrombocytopenia.
Next, the researchers developed a novel anti-CLL-1-ADC, with a highly potent pyrrolobenzodiazepine (PBD) dimer conjugated through a self-immolative disulfide linker. The efficacy and safety profiles of this anti-CLL-1 ADC was evaluated in mouse xenograft models and in Cynomolgus monkeys. Efficacy of this ADC was first evaluated in three xenograft tumor models. These models expressed CLL-1 at levels within the range observed in patients with AML and harbor cytogenetic features of AML patients that typically have a poor prognosis. Compared to controls, anti-CLL-1 ADC demonstrated clear dose-dependent inhibition of tumor growth in mouse models.
Target-dependent tolerability and toxicity of anti-CLL-1 ADC were evaluated in Cynomolgus monkeys. Monkeys received a single IV dose of 0.1 and 0.2 mg/kg of anti-CLL1 ADC, which was tolerated. A slight decrease in the numbers of granulocytes (neutrophils, eosinophils, basophils) and monocytes was observed which reached a limit on days 8–12 after treatment and recovered at day 14/15 (monocytes and basophils) or day 22 (neutrophils). Importantly, cells that lack CLL-1 expression, such as lymphocytes were unaffected by treatment with ant-CLL-1 ADC.
In summary, the anti-CLL-1 ADC is “highly effective at depleting tumors cells in AML xenograft models and lacks target independent toxicities at doses that depleted target myeloid cells in Cynomolgus monkeys”. The researchers concluded that the data from their study suggests that anti-CLL-1 ADC has a prospect to become an effective treatment for AML in humans and resolve some limitations of the current generation of ADCs.