The outlook for patients with refractory/relapsed acute myeloid leukemia (AML) remains poor, with conventional chemotherapeutic treatments often associated with unacceptable toxicities, including severe infections due to profound myelosuppression. Thus there exists an urgent need for more effective agents to treat AML that confer high therapeutic indices and favorable tolerability profiles. Because of its high expression on leukemic blast and stem cells compared with normal hematopoietic stem cells and progenitors, CD123 has emerged as a rational candidate for molecularly targeted therapeutic approaches in this disease. Here we describe the development and preclinical characterization of a CD123-targeting antibody-drug conjugate (ADC), IMGN632, that comprises a novel humanized anti-CD123 antibody G4723A linked to a recently reported DNA mono-alkylating payload of the indolinobenzodiazepine pseudodimer (IGN) class of cytotoxic compounds. The activity of IMGN632 was compared with X-ADC, the ADC utilizing the G4723A antibody linked to a DNA crosslinking IGN payload. With low picomolar potency, both ADCs reduced viability in AML cell lines and patient-derived samples in culture, irrespective of their multidrug resistance or disease status. However, X-ADC exposure was >40-fold more cytotoxic to the normal myeloid progenitors than IMGN632. Of particular note, IMGN632 demonstrated potent activity in all AML samples at concentrations well below levels that impacted normal bone marrow progenitors, suggesting the potential for efficacy in AML patients in the absence of or with limited myelosuppression. Furthermore, IMGN632 demonstrated robust antitumor efficacy in multiple AML xenograft models. Overall, these findings identify IMGN632 as a promising candidate for evaluation as a novel therapy in AML.

The promise of tumor-selective delivery of cytotoxic agents in the form of antibody-drug conjugates (ADC) has now been realized, evidenced by the approval of two ADCs, both of which incorporate highly cytotoxic tubulin-interacting agents, for cancer therapy. An ongoing challenge remains in identifying potent agents with alternative mechanisms of cell killing that can provide ADCs with high therapeutic indices and favorable tolerability. Here, we describe the development of a new class of potent DNA alkylating agents that meets these objectives. Through chemical design, we changed the mechanism of action of our novel DNA cross-linking agent to a monofunctional DNA alkylator. This modification, coupled with linker optimization, generated ADCs that were well tolerated in mice and demonstrated robust antitumor activity in multiple tumor models at doses 1.5% to 3.5% of maximally tolerated levels. These properties underscore the considerable potential of these purpose-created, unique DNAinteracting conjugates for broadening the clinical application of ADC technology. Mol Cancer Ther; 15(8); 1870-8. Ó2016 AACR.

Tumor-selective delivery of cytotoxic agents in the form of antibody-drug conjugates (ADCs) is now a clinically validated approach for cancer treatment. In an attempt to improve the clinical success rate of ADCs, emphasis has been recently placed on the use of DNA-cross-linking pyrrolobenzodiazepine compounds as the payload. Despite promising early clinical results with this class of ADCs, doses achievable have been low due to systemic toxicity. Here we describe the development of a new class of potent DNA-interacting agents wherein changing the mechanism of action from a cross-linker to a DNA alkylator improves the tolerability of the ADC. ADCs containing the DNA alkylator displayed similar in vitro potency, but improved bystander killing and in vivo efficacy, compared to those of the cross-linker. Thus, the improved in vivo tolerability and anti-tumor activity achieved in rodent models with ADCs of the novel DNA alkylator could provide an efficacious, yet safer option for cancer treatment.on May 9, 2018.

Antibody-drug conjugates are an emerging class of cancer therapeutics constructed from monoclonal antibodies conjugated with small molecule effectors. First-generation molecules of this class often employed heterogeneous conjugation chemistry, but many site-specifically conjugated ADCs have been described recently.Here, we undertake a systematic comparison of ADCs made with the same antibody and the same macrocyclic maytansinoid effector but conjugated either heterogeneously at lysine residues or site-specifically at cysteine residues. Characterization of these ADCs in vitro reveals generally similar properties, including a similar catabolite profile, a key element in making a meaningful comparison of conjugation chemistries. In a mouse model of cervical cancer, the lysineconjugated ADC affords greater efficacy on a molar payload basis. Rather than making general conclusions about ADCs conjugated by a particular chemistry, we interpret these results as highlighting the complexity of ADCs and the interplay between payload class, linker chemistry, target antigen, and other variables that determine efficacy in a given setting.

Spleen tyrosine kinase (SYK) is a key activator of signaling pathways downstream of multiple surface receptors implicated in asthma. SYK function has been extensively studied in mast cells downstream of the high-affinity IgE receptor, FcεR1. Preclinical studies have demonstrated a role for SYK in models of allergic inflammation, but a role in airway constriction has not been demonstrated. Here, we have used a potent and selective pharmacological inhibitor of SYK to determine the role of SYK in allergen-mediated inflammation and airway constriction in preclinical models. Attenuation of allergic airway responses was evaluated in a rat passive anaphylaxis model and rat and sheep inhaled allergen challenge models, as well as an ex vivo model of allergen-mediated airway constriction in rats and cynomolgus monkeys. Pharmacological inhibition of SYK dose-dependently blocked IgE-mediated tracheal plasma extravasation in rats. In a rat ovalbumin-sensitized airway challenge model, oral dosing with an SYK inhibitor led to a dose-dependent reduction in lung inflammatory cells. Ex vivo analysis of allergen-induced airway constriction in ovalbumin-sensitized brown Norway rats showed a complete attenuation with treatment of a SYK inhibitor, as well as a complete block of allergen-induced serotonin release. Similarly, allergen-mediated airway constriction was attenuated in ex vivo studies from nonhuman primate lungs. Intravenous administration of an SYK inhibitor attenuated both early- and late-phase allergen-induced increases in airway resistance in an Ascaris-sensitive sheep allergen challenge model. These data support a key role for SYK signaling in mediating allergic airway responses.