Over the last 40 years treatment of acute myeloid leukemia or AML has not significantly changed.

Today, the standard of care for AML is treatment with cytarabine, also known as cytosine arabinoside or ara-C and anthracycline‑based chemotherapy induction regimens including drug such as daunorubicin (daunomycin) or idarubicin and sometimes a third drug, cladribine (Leustatin, 2-CdA).

The re-approval of gemtuzumab ozogamicin (Mylotarg®; Pfizer), an anti-CD33-calicheamicin antibody-drug conjugate or (ADC), has demonstrated that targeted agents like ADCs offer a clinically validated option to enhance the effectiveness of induction therapy.

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Gemtuzumab ozogamicin targets CD33, a 67-kDa type I transmembrane receptor. Based on its broad expression on AML blasts, is target seems appropriate. However, researchers have found that the expression of CD33 on normal hematopoietic stem cells (HSCs) indicates that prolonged cytopenias among multiple hematopoietic lineages may limit the clinical benefit of CD33 targeting agents.

C-type lectin-like molecule-1
Now, following the initial success of gemtuzumab ozogamicin, researchers at Genentech interested in developing a next generation of ADCs for the treatment of AML, are investigating the expression pattern of C-type lectin-like molecule-1 or CLL-1 and its hematopoietic potential were investigated.[1]

They developed a novel anti-CLL-1-ADC which includes a highly potent pyrrolobenzodiazepine (PBD) dimer conjugated through a self-immolative disulfide linker. The efficacy and safety profiles of this ADC were evaluated in mouse xenograft models and in cynomolgus monkeys.

The results of their experimental development were published in the June 2018 issue of Clinical Cancer Research,

As part of their research process, Bing Zheng, Shang-Fan Yu and colleagues first validated CLL-1, a leukemic stem cell marker as a target for AML treatment as compared to CD33.

The researchers analyzed the surface expression of CLL-1 and CD33 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. Their analysis of CLL-1 on bone marrow samples from patients with AML demonstrated expression on CD34 AML blasts but not on lymphocytes or platelets.

Furthermore, CLL-1 expression was also not detected on CD34+/CD38- stem cells obtained from healthy donors. This shows, according to the researchers, that CLL-1 shares similar prevalence and trafficking properties that make CD33 an effective target for AML. But more importantly, CLL-1 lacks the expression of hematopoietic stem cells that may be hampering current CD33 and CD123 targeted ADCs.

Hematopoietic potential
Next, the researchers evaluated the hematopoietic potential of CLL-1. 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).

The study shows that CLL-1 was expressed on at least half of the population of CD34+/CD38+/CD10-/CD45RA-/CD135+ common myeloid progenitor (CMP) cells. CLL-1 was also expressed in 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, as the researchers explain, an early myeloid lineage marker that resides during the differentiation from CMP to GMP cells.

Using colony forming assay, also known as clonogenic assay, the researchers showed that depletion of CD34+/CLL1+ progenitor cells does not impair the normal hematopoietic potential or induce persistent thrombocytopenia.

Conjugation
During the next step in their research, the researchers developed a novel anti-CLL-1-ADC, with a highly potent pyrrolobenzodiazepine (PBD) dimer conjugated through a self-immolative disulfide linker.

They evaluated the efficacy and safety profiles of their anti-CLL-1 ADC in three mouse xenograft tumor models and in Cynomolgus monkeys. These models expressed CLL-1 at levels within the range observed in patients with AML, with cytogenetic features of the disease typically shown in patients with a poor prognosis. In compared to controls, the anti-CLL-1 ADC demonstrated clear dose-dependent inhibition of tumor growth in mouse models.

Finally, the researchers evaluated the target-dependent tolerability and toxicity of anti-CLL-1 ADC in Cynomolgus monkeys.

A single IV dose of 0.1 and 0.2 mg/kg of anti-CLL1 ADC, was tolerated.  The results showed a slight decrease in the numbers of granulocytes (neutrophils, eosinophils, basophils) and monocytes. This was reached a limit on days 8–12 after treatment and recovered at day 14/15 (monocytes and basophils) or day 22 (neutrophils). A key observation was that cells that lack CLL-1 expression, including lymphocytes, were unaffected by treatment with the novel anti-CLL-1 ADC.

The data suggest that a novel anti-CLL-1 ADC may offers the potential of becoming a highly effective, and safer treatment option for patients with AML. By reducing and allowing for faster recovery from initial cytopenias, the novel treatment option resolves some limitations of the current generation of antibody-drug conjugates.