Spotlight pubs.acs.org/crt
■
EFFICACY OF CANCER DRUGS: LOOKING BEYOND THE IC50 How effective is a drug? Scientists answer this question in terms of the drug’s potency, or IC50 (the half-maximum inhibitory concentration). However, the practice of correlating efficacy to IC50 does not take into account the slope of the dose−response curve (Hill slope, HS) or the maximum effect (Emax) of the drug, among other parameters. Even if a drug has a low IC50, it may not necessarily produce the desired effects at reasonable doses in patients. For example, a drug may only kill 50% of cells even at the highest doses (Emax = 0.5), or a drug could have a shallow dose−response curve (HS < 1), meaning that the therapeutic benefit may not increase substantially as the maximum tolerated dose is approached. A research team led by Peter Sorger recently performed multiparametric analysis of IC50, HS, and Emax values for 64 anticancer drugs (comprising 10 different drug classes) tested over 53 established breast cell lines to uncover correlations between efficacy parameters, drug classes, and cell types ((2013) Nat. Chem. Biol., 9, 708−714). For example, they found that Emax strongly correlated to cell type, IC50 and Emax correlated to cell doubling time, and IC50, HS, and Emax correlated to drug class. Overall, the Emax was the best indicator of efficacy at high drug concentrations, while IC50 was more relevant at medium doses. In light of these results, the authors caution that parameters other than IC50 should be used to compare drug efficacy, especially when the drugs are administered at concentrations near and above their IC50. Heidi A. Dahlmann
■
SUBSTRATE-CATALYZED DEPURINATION IN DNA GLYCOSYLASE ALKD
nucleoside, AlkD-catalyzed hydrolysis was 15-fold lower than when 7-Me-dG was next to a normal phosphodieseter-linked nucleoside. Since the crystal structures indicated that the protein does not make any contact with its substrate in the active site, AlkD is a unique example of an enzyme that contains an active site composed exclusively of DNA atoms. Heidi A. Dahlmann
■
TURNING THE TABLES ON PARP INHIBITOR RESISTANCE
Reprinted from Rubinson et al. (2013) Biochemistry, 52, 7363. Copyright 2013 American Chemical Society.
When genotoxic compounds attack DNA, positively charged adducts such as 7-methyldeoxyguanosine (7-Me-dG) can be formed. Although these electronegative adducts are prone to spontaneous hydrolysis, which generates abasic sites in DNA, DNA glycosylases catalyze the reaction as well. Many glycosylases contain carboxylate side chains that either stabilize the oxocarbenium intermediate generated during depurination or activate the nucleophilic water molecule that subsequently attacks the oxocarbenium ion. In the case of uracil DNA glycosylase, DNA phosphate groups also have an effect. Furthermore, according to a recent report by Brandt Eichman and co-workers, DNA glycosylase AlkD distorts the DNA backbone such that its own phosphate groups seem to exclusively fill the role of stabilizing the depurination intermediate ((2013) Biochemistry, 52, 7363−7365). The research team obtained crystal structures showing that AlkD flipped a model abasic site out of the double helix and induced a distortion to the DNA backbone that brought the deoxyribose substrate into proximity with the negatively charged 3′-phosphate of the adjacent nucleoside. Hypothesizing that the DNA backbone played a role in catalyzing 7-Me-dG depurination, the team screened the efficiency of AlkDmediated 7-Me-dG depurination of DNA duplexes containing neutral methylphosphonate substitutions at various phosphodieseter linkages in the DNA backbone. They found that when 7-Me-dG was adjacent to a methylphosphonate-substituted © 2013 American Chemical Society
When DNA becomes damaged, either spontaneously or due to anticancer drugs, single-strand (SS) DNA breaks may form. SS breaks can be repaired by pathways involving proteins such as poly(ADP-ribose) polymerase (PARP), but SS breaks that evade repair can be converted into cytotoxic double-strand (DS) breaks. DS breaks can also be repaired through a process called homologous recombination (HR), which is mediated by breast cancer type 1 susceptibility protein (BRCA1). PARP inhibitors are an ideal means to attack breast cancers that have reduced ability to perform HR, particularly those deficient in BRCA1. However, BRCA1 mutant cancers are Published: December 16, 2013 1776
dx.doi.org/10.1021/tx400426k | Chem. Res. Toxicol. 2013, 26, 1776−1777
Chemical Research in Toxicology
Spotlight
scorpion venom peptides, to the extent that the venom even acts as an analgesic against other painful irritants such as formalin. When the research team compared Nav8 from the grasshopper mouse with that from house mice (which are sensitive to bark scorpion venom), they identified multiple amino acid variations between the two proteins. These variations occurred primarily in the extracellular loops and pore of the channel, i.e., sites which are accessible to venom peptides. The authors note that while Nav8 is not normally targeted by bark scorpion venom, the unique amino acid variants found in grasshopper mouse Nav8 facilitate binding of the venom, which causes subsequent Na+ current inhibition. Heidi A. Dahlmann
extremely susceptible to developing resistance to PARP inhibitors through a variety of mechanisms, including secondary mutations in BRCA1 that restore its function. Alternatively, as recently reported by Neil Johnson, Geoffrey Shapiro, and colleagues, some mutant forms of BRCA1 regain function through stabilization by heat shock protein (HSP)90 ((2013) PNAS, 110, 17041−17046). The research team discovered that upon treating PARP inhibitor-resistant cells with an HSP90 inhibitor, they became sensitive to PARP inhibition. The authors note that supplementing PARP-inhibitor-based treatment with HSP90 inhibitors may prove useful for treating recalcitrant BRCA1 mutant cancers. Heidi A. Dahlmann
■
SLEEP DRIVES METABOLITE CLEARANCE FROM THE ADULT BRAIN Sleep deprivation is rough on the brain, with reduced ability to concentrate and a prolonged reaction time being among the most common side effects and an increased susceptibility to seizures, dementia, and death occurring in the most extreme circumstances. Researchers have long pondered the restorative nature of sleep (and the dire consequences of a lack thereof). A recent report by Maiken Nedergaard and co-workers ((2013) Science, 342, 373−377) sheds light on the matter from both physiological and molecular perspectives. Excess proteins, such as neurodegenerative disease-linked β-amyloid protein (Aβ) and metabolic waste products, are cleared from the interstitial space surrounding brain cells through the interchange of interstitial fluid with cerebrospinal fluid. The observation that Aβ levels were higher in wakeful than in sleeping rodents and humans prompted Nedergaard and co-workers to hypothesize that Aβ clearance is increased during sleep. They found that both sleeping and anesthetized mice had increased interstitial volumes than did awake mice, which resulted in faster clearance of radiolabled Aβ. The authors speculate that the restorative function of sleep is derived from the facilitated clearance of potentially toxic CNS waste products that accumulate during wakefulness. Heidi A. Dahlmann
■
VOLTAGE-GATED SODIUM CHANNEL DEFENDS GRASSHOPPER MICE FROM BARK SCORPION TOXIN
For animals, pain is anything but pleasant, but it serves the important purpose of warning of tissue damage. Certain animals use the element of pain to their advantage by producing pain-eliciting venoms to defend themselves from predators. For bark scorpions, this defense-mechanism backfires when it comes to the grasshopper mice (O. torridus) that prey on them. As described by Ashlee Rowe and co-workers ((2013) Science, 342, 441−446), the grasshopper mouse Nav8 sodium channel, which normally transmits pain signals, is actually inhibited by the 1777
dx.doi.org/10.1021/tx400426k | Chem. Res. Toxicol. 2013, 26, 1776−1777
