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Purpose Despite the availability of new drugs, many patients with acute myeloid leukemia (AML) do not achieve remission and outcomes remain poor. Venetoclax is a promising new therapy approved for use in combination with a hypomethylating agent or with low‐dose cytarabine for the treatment of newly diagnosed older AML patients or those ineligible for intensive chemotherapy. 225Actinium‐lintuzumab (225Ac‐lintuzumab) is a clinical stage radioimmunotherapy targeting CD33 that has shown evidence of single‐agent activity in relapsed/refractory AML. Increased expression of MCL‐1 is a mediator of resistance to venetoclax in cancer. Experimental design Here we investigated the potential for 225Ac‐lintuzumab‐directed DNA damage to suppress MCL‐1 levels as a possible mechanism of reversing resistance to venetoclax in two preclinical in vivo models of AML. Results We demonstrated that 225Ac‐lintuzumab in combination with venetoclax induced a synergistic increase in tumor cell killing compared to treatment with either drug alone in venetoclax‐resistant AML cell lines through both an induction of double‐stranded DNA breaks (DSBs) and depletion of MCL‐1 protein levels. Further, this combination led to significant tumor growth control and prolonged survival benefit in venetoclax‐resistant in vivo AML models. Conclusions There results suggest that the combination of 225Ac‐lintuzumab with venetoclax is a promising therapeutic strategy for the treatment of patients with venetoclax‐resistant AML. Clinical trial of this combination therapy (NCT03867682) is currently ongoing., Long‐term survival of adult patients with acute myeloid leukemia (AML) remains unsatisfactory.We describe the results of a mechanistic and efficacy study combining venetoclax (ABT‐199), a recently approved BCL2 inhibitor, and the clinical stage CD33‐targeting antibody radiolabeled with 225Actinium (225Ac‐lintuzumab) in two venetoclax‐resistant AML models in vitro and in vivo. The study demonstrated dramatic synergy of these two drugs both in vitro and in vivo due to the decrease in BCL‐2 and MCL‐1 levels and induction of double DNA strand breaks. Since both venetoclax and 225Ac‐lintuzumab are already used in patients, we anticipate that rapid translation of this combination approach into the clinic will benefit patients with AML.

Summary Melanin is a ubiquitous pigment with unique physicochemical properties. The resistance of melanized fungi to cosmic and terrestrial ionizing radiation suggests that melanin also plays a pivotal role in radioprotection. In this study, we compared the effects of densely-ionizing deuterons and sparsely-ionizing X-rays on two microscopic fungi capable of melanogenesis. We utilized the fast-growing pathogenic basiodiomycete forming an induced DOPA-melanin, Cryptococcus neoformans (CN); and the slow-growing environmental rock-inhabiting ascomycete synthesizing a constitutive DHN-melanin, Cryomyces antarcticus (CA); melanized and non-melanized counterparts were compared. CA was more resistant to deuterons than CN, and similar resistance was observed for X-rays. Melanin afforded protection against high-dose (1.5 kGy) deuterons for both CN and CA (p-values

Melanized microorganisms are often found in environments with very high background radiation levels such as in nuclear reactor cooling pools and the destroyed reactor in Chernobyl. These findings and the laboratory observations of the resistance of melanized fungi to ionizing radiation suggest a role for this pigment in radioprotection. We hypothesized that the radioprotective properties of melanin in microorganisms result from a combination of physical shielding and quenching of cytotoxic free radicals. We have investigated the radioprotective properties of melanin by subjecting the human pathogenic fungi Cryptococcus neoformans and Histoplasma capsulatum in their melanized and non-melanized forms to sublethal and lethal doses of radiation of up to 8 kGy. The contribution of chemical composition, free radical presence, spatial arrangement, and Compton scattering to the radioprotective properties of melanin was investigated by high-performance liquid chromatography, electron spin resonance, transmission electron microscopy, and autoradiographic techniques. Melanin protected fungi against ionizing radiation and its radioprotective properties were a function of its chemical composition, free radical quenching, and spherical spatial arrangement.

The capsule of Cryptococcus neoformans can undergo dramatic enlargement, a phenomenon associated with virulence. A prior study that used Ab to the capsule as a marker for older capsular material concluded that capsule growth involved the intermixing of new and old capsular material with displacement of older capsular polysaccharide towards the surface. Here we have revisited that question using complement (C), which binds to capsular polysaccharide covalently, and cannot redistribute by dissociation and binding at different sites. The experimental approach involved binding of C to cells with small capsules, inducing capsule growth, and following the location of C relative to the cell wall as the capsule enlarged. C remained close to the cell wall during capsule growth, indicating that capsule enlargement occurred by addition of new polysaccharide near the capsule edge. This conclusion was confirmed by an independent method that employed radioactive metabolic labelling of newly synthesized capsule with 3H-mannose followed by gradual capsular stripping with gamma-radiation. Capsule growth proceeded to a certain size, which was a function of cell size, and was not degraded when the cells were transferred to a non-inducing medium. During budding, an opening appeared in the capsule of the mother cell that permitted the nascent bud to separate. Scanning EM suggested that a physical separation formed between the capsules of the mother and daughter cells during budding, which may avoid mixture between both capsules. Our results indicate that C. neoformans capsular enlargement also occurs by apical growth and that budding results in capsular rearrangements.