Eeded, for example, during wound healing (Demaria et al., 2014). This possibility merits further study in animal models. Additionally, as senescent cells do not divide, drug resistance would journal.pone.0158910 be expected to be less likely journal.pone.0158910 be expected to be less likely pnas.1602641113 than is the case with antibiotics or cancer treatment, in whichcells proliferate and so can acquire resistance (Tchkonia et al., 2013; Kirkland Tchkonia, 2014). We view this work as a first step toward developing senolytic treatments that can be administered safely in the clinic. Several issues remain to be addressed, including some that must be examined well before the agents described here or any other senolytic agents are considered for use in humans. For example, we found differences in responses to RNA interference and senolytic agents among cell types. Effects of age, type of disability or disease, whether senescent cells are continually generated (e.g., in diabetes or high-fat diet vs. effects of a single dose of radiation), extent of DNA damage responses that accompany senescence, sex, drug metabolism, immune function, and other interindividual differences on responses to senolytic agents need to be studied. Detailed testing is needed of many other potential targets and senolytic agents and their combinations. Other dependence receptor networks, which promote apoptosis unless they are constrained from doing so by the presence of ligands, might be particularly informative to study, especially to develop cell type-, tissue-, and disease-specific senolytic agents. These receptors include the insulin, IGF-1, androgen, and nerve growth factor receptors, among others (Delloye-Bourgeois et al., 2009; Goldschneider Mehlen, 2010). It is possible that more existing drugs that act against the targets identified by our RNA interference experiments may be senolytic. In addition to ephrins, other dependence receptor ligands, PI3K, AKT, and serpines, we anticipate that drugs that target p21, probably p53 and MDM2 (because they?2015 The Authors. Aging Cell published by the Anatomical Society and John Wiley Sons Ltd.Senolytics: Achilles’ heels of senescent cells, Y. Zhu et al.(A)(B)(C)(D)(E)(F)Fig. 6 Periodic treatment with D+Q extends the healthspan of progeroid Ercc1?D mice. Animals were treated with D+Q or vehicle weekly. Symptoms associated with aging were measured biweekly. Animals were euthanized after 10?2 weeks. N = 7? mice per group. (A) Histogram of the aging score, which reflects the average percent of the maximal symptom score (a composite of the appearance and severity of all symptoms measured at each time point) for each treatment group and is a reflection of healthspan (Tilstra et al., 2012). *P < 0.05 and **P < 0.01 Student's t-test. (B) Representative graph of the age at onset of all symptoms measured in a sex-matched sibling pair of Ercc1?D mice. Each color represents a different symptom. The height of the bar indicates the severity of the symptom at a particular age. The composite height of the bar is an indication of the animals' overall health (lower bar better health). Mice treated with D+Q had delay in onset of symptoms (e.g., ataxia, orange) and attenuated expression of symptoms (e.g., dystonia, light blue). Additional pairwise analyses are found in Fig. S11. (C) Representative images of Ercc1?D mice from the D+Q treatment group or vehicle only. Splayed feet are an indication of dystonia and ataxia. Animals treated with D+Q had improved motor coordination. Additional images illustrating the animals'.