Discovery, Characterization, and Structure-Based Optimization of Small-Molecule In Vitro and In Vivo Probes for Human DNA Polymerase Theta

Human DNA polymerase theta (Polθ), which is essential for microhomology-mediated DNA double strand break repair, has been proposed as an attractive target for the treatment of BRCA deficient and other DNA repair pathway defective cancers. As previously reported, we recently identified the first selective small molecule Polθ in vitro probe, 22 (ART558), which recapitulates the phenotype of Polθ loss, and in vivo probe, 43 (ART812), which is efficacious in a model of PARP inhibitor resistant TNBC in vivo. Here we describe the discovery, biochemical and biophysical characterization of these probes including small molecule ligand co-crystal structures with Polθ. The crystallographic data provides a basis for understanding the unique mechanism of inhibition of these compounds which is dependent on stabilization of a "closed" enzyme conformation. Additionally, the structural biology platform provided a basis for rational optimization based primarily on reduced ligand conformational flexibility.

Polθ inhibitors elicit BRCA-gene synthetic lethality and target PARP inhibitor resistance

To identify approaches to target DNA repair vulnerabilities in cancer, we discovered nanomolar potent, selective, low molecular weight (MW), allosteric inhibitors of the polymerase function of DNA polymerase Polθ, including ART558. ART558 inhibits the major Polθ-mediated DNA repair process, Theta-Mediated End Joining, without targeting Non-Homologous End Joining. In addition, ART558 elicits DNA damage and synthetic lethality in BRCA1- or BRCA2-mutant tumour cells and enhances the effects of a PARP inhibitor. Genetic perturbation screening revealed that defects in the 53BP1/Shieldin complex, which cause PARP inhibitor resistance, result in in vitro and in vivo sensitivity to small molecule Polθ polymerase inhibitors. Mechanistically, ART558 increases biomarkers of single-stranded DNA and synthetic lethality in 53BP1-defective cells whilst the inhibition of DNA nucleases that promote end-resection reversed these effects, implicating these in the synthetic lethal mechanism-of-action. Taken together, these observations describe a drug class that elicits BRCA-gene synthetic lethality and PARP inhibitor synergy, as well as targeting a biomarker-defined mechanism of PARPi-resistance.

Effect of hypercholesterolemia on Ca(2+)-dependent K(+) channel-mediated vasodilatation in vivo

Nitric oxide (NO)-mediated and NO-independent mechanisms of endothelium-dependent vasodilatation involve Ca(2+)-dependent K(+) (K(Ca)) channels. We examined the role in vivo of K(Ca) channels in NO-independent vasodilatation in hypercholesterolemia. Hindlimb vascular conductance was measured at rest and after aortic injection of ACh, bradykinin (BK), and sodium nitroprusside in anesthetized control and cholesterol-fed rabbits. Conductances were measured before and after treatment with the NO synthase antagonist N(omega)-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg) or K(Ca) blockers tetraethylammonium (30 mg/kg), charybdotoxin (10 microgram/kg), and apamin (50 microgram/kg). The contribution of NO to basal conductance was greater in control than in cholesterol-fed rabbits [2.2 +/- 0.4 vs. 1.1 +/- 0.3 (SE) ml. min(-1). kg(-1). 100 mmHg(-1), P < 0.05], but the NO-independent K(Ca) channel-mediated component was greater in the cholesterol-fed than in the control group (1.1 + 0.4 vs. 0.3 +/- 0.1 ml. min(-1). kg(-1). 100 mmHg(-1), P < 0.05). Maximum conductance response to ACh and BK was less in cholesterol-fed than in control rabbits, and the difference persisted after L-NAME (ACh: 7.7 +/- 0.7 vs. 10.1 +/- 0.5 ml. min(-1). kg(-1). 100 mmHg(-1), P < 0.005). Blockade of K(Ca) channels with tetraethylammonium or charybdotoxin + apamin almost completely abolished L-NAME-resistant vasodilatation after ACh or BK. The magnitude of K(Ca)-mediated vasodilatation after ACh or BK was impaired in hypercholesterolemic rabbits. Vasodilator responses to nitroprusside did not differ between groups. In vivo, hypercholesterolemia is associated with an altered balance between NO-mediated and NO-independent K(Ca) channel contributions to resting vasomotor tone and impairment of both mechanisms of endothelium-dependent vasodilatation.

Effects of dietary L-arginine on atherosclerosis and endothelium-dependent vasodilatation in the hypercholesterolemic rabbit. Response according to treatment duration, anatomic site, and sex

BACKGROUND

Nitric oxide (NO) may protect arteries against atherosclerosis. In the present study, we examined whether dietary L-arginine, the precursor of NO, could chronically preserve endothelium-dependent vasodilatation in vivo and/or limit atherogenesis.

METHODS AND RESULTS

Rabbits were randomized according to sex to receive 2% dietary cholesterol, with or without L-arginine (2.25% solution), for 7 or 14 weeks. Hindlimb vasodilator responses to acetylcholine and nitroprusside were measured with an electromagnetic flow probe. Atherosclerosis was measured with planimetry of aortic lesions stained with Oil-Red-O. In rabbits administered L-arginine, plasma arginine levels increased to 483 +/- 30 mumol/L at 3 weeks (mean +/- SEM, P < .0001 versus control animals) but declined to 224 +/- 25 mumol/L at 7 weeks (P = .02) and to 100 +/- 23 mumol/L at 14 weeks (NS versus control animals). At 7 weeks, peak hindlimb conductance in response to acetylcholine in cholesterol-fed males was 249 +/- 49% of baseline compared with 332 +/- 9% in control animals (P = .04), but peak response in arginine-fed rabbits (314 +/- 24%) did not differ from that of control animals. At 14 weeks, peak responses to acetylcholine were equally reduced in males fed cholesterol with (266 +/- 21%, P = .02 versus control) or without (263 +/- 13%, P = .01 versus control) L-arginine. Similar impairment of endothelium-dependent vasodilatation was seen in females at 14 weeks. Vasodilator responses to nitroprusside did not differ from those of control animals in any treatment group. After 14 weeks, atherosclerosis was less in the descending aorta of arginine-fed males (16 +/- 4% surface area) than that of males fed cholesterol only (42 +/- 8%, P = .04), but no treatment benefit was seen in the ascending aorta or in females.

CONCLUSIONS

Dietary L-arginine supplementation causes an early rise in plasma arginine levels, with limitation of atherosclerosis in the descending aorta and preservation of endothelium-dependent vasodilatation in resistance arteries, but this treatment effect is not sustained. Dietary L-arginine may not be of long-term benefit in the prevention of atherosclerosis in humans.

Relation between age, arterial distensibility, and aortic dilatation in the Marfan syndrome

This study examined the relations between age, arterial distensibility, and systemic hemodynamics in patients with the Marfan syndrome. The study group included 170 patients referred to a specialist clinic, of whom 55 (age 26 +/- 12 years) were diagnosed as having Marfan syndrome. The remaining 115 patients (age 25 +/- 14 years) formed a control group. Each patient underwent echocardiographic examination, with measurement of ascending aorta diameter at end-diastole and end-systole, and aortic flow velocities. The elastic properties of the aorta were indexed by calculation of aortic distensibility, wall stiffness, and systemic pulse wave velocity. Mean end-diastolic aortic diameter in the Marfan group (38 +/- 9 mm) was greater than that in the controls (26 +/- 4 mm, p < 0.01). Resting heart rate and aortic flow velocities were similar in the 2 groups, but systemic arterial pulse pressure was greater in the Marfan group (50 +/- 12 mm Hg) than in the controls (41 +/- 8 mm Hg, p < 0.01). Aortic diameter increased with age in both groups, but at all ages the Marfan group exhibited greater aortic diameters (p < 0.05). Aortic distensibility was less in the Marfan group (2.6 +/- 1.3 cm2.dynes-1 x10(-6)) than in the controls (6.2 +/- 2.1 cm2.dynes-1 x 10(-6), p < 0.01), and the aortic wall stiffness index was greater in the Marfan group (7.9 +/- 3.4) than in the controls (2.8 +/- 0.6, p < 0.01). Aortic wall stiffness increased with age and aortic diameter, but at all ages the Marfan group exhibited a stiffer aorta for a given diameter than did the controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlates of torpor in the insectivorous dasyurid marsupial Sminthopsis murina

Body temperatures, oxygen consumption and the thermal response of liver mitochondrial membranes of the Common Dunnart, Sminthopsis murina, were measured during winter and the summer mating season. In winter body temperatures for S. murina during spontaneous torpor were as low as 15oC and oxygen consumption was reduced to 6% of normothermic resting animals. In summer animals remained normothermic. For the winter animals liver mitochondrial succinate oxidase and succinate: cytochrome c reductase showed a constant apparent Arrhenius activation energy (Ea) over the temperature range 6 to 40oC. For summer animals Ea increased below about 20oC. The temperature coefficient for the motion of spin labels intercalated with membrane lipids increased below about 8oC for membranes from winter animals and below 20oC for summer animals.