Highest Non-severely Toxic Dose Research Articles

An FDA oncology analysis of toxicities associated with PBD-containing antibody-drug conjugates

With a new generation of antibody-drug conjugates (ADCs) that contain a drug-to-antibody ratio (DAR) of 2, the question remains whether advances in technology have resulted in more stable and tumor-specific ADCs. These ADCs are anticipated to cause minimal systemic exposures of payloads, with toxicities being evident mainly at tumor sites. We examined 15 ADCs with PBD-dimer payloads and a DAR of 2 and concluded that dose limiting toxicities in animals and in humans are generally related to the payload. Both the payloads and the ADCs had pro-inflammatory responses causing severe toxicities that were at times of low incidence, making it difficult to assess a cause-effect relationship. Due to their low incidence, single-patient cohorts may not detect these events and such design may not be suitable in first-in-human (FIH) trials. The commonly proposed approach by the sponsors for FIH dose selection was 1/6th highest non-severely toxic dose (HNSTD) in monkeys. This approach resulted in an acceptable balance of safety and efficient dose escalation in phase 1 trials, when using data from repeat-dose toxicology studies and body surface area for scaling. No sponsor used the data generated in rodents or proposed novel approaches for FIH dose selection.

Abstract 3901: Nonclinical safety evaluation of CA102N, a hyaluronic acid (HA) and nimesulide (Nim) conjugate, for colorectal cancer treatment

Abstract CA102N is a conjugate of the biological polymer hyaluronic acid (HA) and Nimesulide (Nim), a cyclooxygenase-2 (COX-2) inhibitor that represents a promising new approach to addressing cancer treatment. Similar to an antibody-drug conjugate or a pro-drug, CA102N enhances the efficacy of Nim through targeting and binding of HA to CD44 receptors highly expressed in solid tumors. Pharmacological (in vitro and in vivo) studies indicated that CA102N exhibits apoptotic and tumor growth inhibitory activities against colorectal cancer. The MOA of CA102N was demonstrated to be related to apoptosis and disruption of the PI3K/Akt/mTOR pathway in addition to inhibition of the COX-2/PGE2 related pathways. Safety of CA102N was assessed in acute dose range finding and repeat-dose intravenous toxicity studies in rats and beagle dogs. Three dose levels including the Maximum Tolerated Dose (MTD) from DRF study, a middle dose and a low dose equivalent to the proposed clinical dose were administered twice weekly (BID). A 28-day recovery group was also included in the rat study. Overall CA102N was well tolerated in both species. CA102N did not induce any significant effects on the CNS, the cardiovascular and respiratory systems in animal models. Some changes in hematological parameters were observed only in rats at the highest dose (Std 10: ≥ 600 mg/kg/day ), however, most of these observations were reversible. No hERG inhibition (in vitro) was observed suggesting the risk of QT prolongation with CA102N is low. The Highest Non-Severely Toxic Dose (HNSTD) of CA102N in the dog was determined to be 280 mg/kg/day (7.24 mg/kg in Nim equivalents ) and the rat NOAEL was 400 mg/kg/day (10.16 mg/kg in Nim equivalents ). These doses are 2.3 times (rat) and 5.6 times (dog) of nimesulide dose to be evaluated (up to 0.72 mg/kg of nimesulide) in the proposed phase 1 study. On the basis of the mechanism of action and the nonclinical safety profiles, toxicity associated risks to patients appear to be low at the proposed clinical dose levels. A Phase I study of CA102N in patients with locally advanced or metastatic colorectal cancer is ongoing. Citation Format: Eskouhie Tchaparian, Louis Lin, Elin Hsu, Ivy Chen, Albert Wu. Nonclinical safety evaluation of CA102N, a hyaluronic acid (HA) and nimesulide (Nim) conjugate, for colorectal cancer treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3901.

Abstract 4121: Preclinical characterization of CS1003, a novel clinical-stage PD-1 monoclonal antibody

Abstract Background: The programmed cell death protein 1 receptor (PD-1) is an immune-checkpoint protein that negatively regulates the immune system to avoid collateral damage to self-tissues. Tumors hijack this mechanism as a way to avoid immune detection and destruction. Presently, the FDA has approved three anti-PD-1 antibodies for the treatment in more than ten cancer types. CS1003 is a novel anti-PD-1 mAb recognizing a unique epitope developed to disrupt the PD-1 interaction with PD-L1/PD-L2 to restore or improve T-cell function as stand-alone therapy or in combination with other approaches. Methods: A panel of rat PD-1 mAbs were generated through conventional hybridoma technology. Several mAbs with favorable characteristics were further humanized. CS1003, a humanized, hinge-stabilized IgG4 mAb, was selected as our lead candidate for further characterization including antigen binding, biophysical properties as well as functional characterization. Results: CS1003 binds mouse, cynomolgus monkey and human PD-1 with similar affinity. CS1003 bound PD-1-expressing cell lines and chronically-activated T cells, blocked PD-1 interactions with PD-L1/PD-L2, resulting in inhibition of PD-1 signaling, and enhanced T cell cytokine secretion and proliferation to levels comparable to those observed with nivolumab or pembrolizumab reference molecules. CS1003 showed significantly anti-tumor activity in conventional MC38 as well as in hPD-1 knock-in mice. CS1003 showed no unexpected cross-reactivity in human tissues, with specific staining observed primarily in lymphocytes of lymphoid organs. In a pharmacokinetic (PK) study in cynomolgus monkeys following single intravenous administration at multiple dose levels, PK properties were linear with the proportionally increasing exposures from 2-18 mg/kg. In a repeated-dose toxicity study, CS1003 was well tolerated in cynomolgus monkeys and the major finding was mononuclear infiltration in multiple organs, which was consistent with its pharmacologic activity. CS1003 demonstrated a favorable safety profile with the highest non-severely toxic dose (HNSTD) of 100 mg/kg. Conclusion: CS1003 is a novel anti-PD-1 mAb with favorable preclinical characteristics and safety profile in cynomolgus monkeys. The unique feature of CS1003 is its cross-reactivity with both human and murine PD-1. This should greatly facilitate further evaluation its potential for combination therapy in various syngeneic mouse tumor models. A phase I study for CS1003 is being conducted to evaluate the safety, tolerability, PK and anti-tumor activity in patients with advanced tumors [NCT03475251]. Citation Format: Fu Li, Jingrong Li, Ke Yin, Juan Zhang, Zhenhu Li, Liang Lu, Yuanwu Bao, Dan Pu, Zhen Qin, Yong Zheng, Baotian Yang, Jing Li, Xinzhong Jon Wang. Preclinical characterization of CS1003, a novel clinical-stage PD-1 monoclonal antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4121.

Abstract 4821: Development of a potent Trop-2 antibody-drug conjugate, BAT8003, for the treatment of Trop-2 positive gastric tumors

Abstract Trophoblast cell surface antigen 2 (Trop-2) is overexpressed on many epithelial carcinomas while having low levels of expression on normal tissue. Overexpression of Trop-2 has been correlated with poor prognosis in several solid tumors. Trop-2 is overexpressed in ~60% of gastric cancer, which causes more than 500,000 mortalities every year in China. We have developed a Trop-2 antibody-drug conjugate (ADC), BAT8003, that utilizes an anti-Trop-2 IgG1 antibody, engineered for site-specific conjugation, a novel uncleavable linker, and a potent maytansine derivative as the payload. Site-specific conjugation allows for a more controllable drug-antibody ratio (DAR), resulting in a more homogenous product. BAT8003 also incorporates increased ADCC effects through afucosylation to further enhance potency. Here we show that BAT8003 is internalized in Trop-2 positive cells. It inhibits proliferation of a number of different Trop2-overexpressed tumor cells with IC50s of ~1 nM. In a gastric cancer (NCI-N87) mouse xenograft model, BAT8003 demonstrates strong inhibition activity on tumor growth at doses of 5 mg/kg and 15 mg/kg. A multiple dose toxicity study in monkey reveals that the highest non-severely toxic dose (HNSTD) of BAT8003 is 20 mg/kg when dose once every 3 weeks. The preclinical profile of BAT8003 warrants further clinical development for the treatment of gastric cancer as well as other Trop-2 over-expressing cancers. Citation Format: Weijia Tang, Xingxing Mei, Ziqiang Ou, Jirong Gan, Shengfeng Li, Jin-Chen Yu. Development of a potent Trop-2 antibody-drug conjugate, BAT8003, for the treatment of Trop-2 positive gastric tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4821.

Abstract LB-A20: Strategies to Improve the Safety and Efficacy of Biparatopic Antibody Drug Conjugates

Abstract Antibody drug conjugates (ADC’s) define an emerging class of targeted therapies for cancer, with over 60 ADC’s currently in clinical development. Although antibodies can exhibit exquisite target specificity, expression of the tumor target in normal tissue can limit the tolerated dose that one can achieve with an ADC. We designed two different affinity variant biparatopic antibodies that recognize distinct epitopes on EGFR and promote receptor clustering that leads to enhanced lysosomal trafficking. These ADC’s comprise two different strategies to target EGFR expressing tumors: a high affinity, potency driven approach and a lowered affinity, threshold driven approach. We show that both ADC’s bind to tumor cells, are rapidly internalized, and traffic to the lysosome, where the warhead is efficiently released by lysosomal hydrolases. In vitro, the high affinity ADC exhibited potent killing of cancer cell lines, with IC50’s in the double digit picomolar range in cells with relative EGFR densities as low as 25,000, while the low affinity ADC was most effective when the EGFR density was greater than 500,000. The high affinity ADC was active in patient derived xenograft (PDX) models derived from colorectal and lung cancer patients harboring clinically relevant oncogenic drivers, such as KRAS and BRAF mutations or EGFR aberrations. This ADC induced tumor stasis or regression, with minimally efficacious doses between 1 and 3 mg/kg, in multiple tumor models across a broad range of EGFR expression (IHC scores ranging from 1 to 3). In a cynomolgus monkey safety study, this ADC showed a manageable safety profile at 2 mg/kg, with skin effects as the dose limiting toxicity. The potency of the high affinity ADC may provide meaningful activity across a range of EGFR expression levels, at doses that are tolerable in the clinic. The lowered affinity ADC exhibited an improved safety profile compared to the high affinity ADC, with a highest non-severely toxic dose (HNSTD) of 6 mg/kg, presumably driven by a reduction in binding to normal tissues that express comparably lower EGFR levels relative to the tumor. This lowered affinity ADC showed a more restricted anti-tumor activity, causing regressions at a minimally efficacious dose between 3 and 5 mg/kg, in PDX models that expressed high levels of EGFR, as judged by IHC scores of 3. The lowered affinity ADC required higher EGFR expression for activity, but the improved tolerability may provide a selectivity threshold that discriminates tumor from normal tissue in patients whose tumors overexpress EGFR. Our findings support the concept that modulating antibody affinity can yield distinct ADC’s that represent different strategies for balancing safety and efficacy. The work presented here provides a conceptual framework for designing unique ADC candidates tailored to the particular expression profile and safety considerations around specific tumor associated targets. Citation Format: Frank I. Comer, Xinzhong Wang, Andy Q. Yuan, Vanessa Muniz-Medina, Agnieszka Sadowska, Lan Xu, Samuel Perry, Pamela Thompson, Ralph Minter, Robert Woods, Mary Jane Hinrichs, Molly Reed, Shameen Affif-Rider, Marlon Rebelatto, Charles Brown, Hui Feng, Rakesh Dixit, Steven Coats, Jane Osbourn. Strategies to Improve the Safety and Efficacy of Biparatopic Antibody Drug Conjugates [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-A20.

Abstract 820: Preclinical development of MGC018, a duocarmycin-based antibody-drug conjugate targeting B7-H3 for solid cancer

Abstract Introduction: B7-H3, a member of the B7 family of immunomodulatory molecules, is overexpressed in a wide range of solid tumors; tumor overexpression has been correlated with disease severity and poor outcome in several cancer types. MGC018 is an antibody-drug conjugate (ADC) targeted against B7-H3 and comprised of the cleavable linker-duocarmycin payload, valine-citrulline-seco DUocarmycin hydroxyBenzamide Azaindole (vc-seco-DUBA), conjugated to an anti-B7-H3 humanized IgG1/kappa monoclonal antibody through reduced interchain disulfides, with an average drug-to-antibody ratio of ~2.7. Previous studies indicated MGC018 exhibited a favorable preclinical profile, with strong reactivity toward tumor cells and tumor-associated vasculature, limited normal tissue reactivity, potent cytotoxicity in vitro and antitumor activity in vivo toward a range of B7-H3-expressing tumor cell lines representing several cancer types. Based on these preliminary results, expanded preclinical development of MGC018 was undertaken to support clinical development. Methods: vc-seco-DUBA conjugation to obtain MGC018 ADC was performed by Synthon Biopharmaceuticals B.V. Single- and repeat-dose in vivo efficacy studies were conducted in CD-1 nude mice with human tumor xenografts that express B7-H3 to explore the relationship between Cmax, exposure and anti-tumor activity, and to define the minimal efficacious dose in these models. A GLP toxicology study was conducted in cynomolgus monkeys in which MGC018 was administered at dose levels of 1, 3, 6 and 10 mg/kg every three weeks for a total of three doses. Results: MGC018 demonstrated specific, dose-dependent in vivo antitumor activity toward B7-H3-positive tumor xenografts representing breast, lung and ovarian cancers, and melanoma. Fractionated MGC018 dose studies were consistent with antitumor activity driven by the total exposure (AUC) rather than peak drug exposure (Cmax). MGC018 was tolerated in cynomolgus monkeys at all dose levels tested, with 10 mg/kg, the highest dose administered, defined as the highest non-severely toxic dose (HNSTD). Conclusion: MGC018, a preclinical candidate comprised of a humanized mAb targeting B7-H3, conjugated to the potent DNA alkylating payload DUBA via a cleavable peptide linker, exhibited a favorable preclinical profile. MGC018 demonstrated potent antitumor activity in vivo toward B7-H3-expressing tumor xenografts at clinically relevant dose levels. MGC018 was tolerated in cynomolgus monkeys, a relevant toxicology model, at exposure levels in excess of those required for antitumor activity. Our findings support the clinical development of MGC018 to evaluate its potential as an ADC therapeutic for B7-H3-expressing solid cancers. Citation Format: Juniper A. Scribner, Jennifer G. Brown, Sharad Sharma, Hua Li, Michael Chiechi, Pam Li, Thomas Son, Anushka De Costa, Yan Chen, Francine Chen, Bhaswati Barat, Ling Huang, Christina Wolff, Jeff Hooley, Tim E. Hotaling, Timur Gaynutdinov, Valentina Ciccarone, James Tamura, Scott Koenig, Syd Johnson, Paul A. Moore, Ezio Bonvini, Deryk Loo. Preclinical development of MGC018, a duocarmycin-based antibody-drug conjugate targeting B7-H3 for solid cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 820.

DS-8201a, A Novel HER2-Targeting ADC with a Novel DNA Topoisomerase I Inhibitor, Demonstrates a Promising Antitumor Efficacy with Differentiation from T-DM1.

An anti-HER2 antibody-drug conjugate with a novel topoisomerase I inhibitor, DS-8201a, was generated as a new antitumor drug candidate, and its preclinical pharmacologic profile was assessed. In vitro and in vivo pharmacologic activities of DS-8201a were evaluated and compared with T-DM1 in several HER2-positive cell lines and patient-derived xenograft (PDX) models. The mechanism of action for the efficacy was also evaluated. Pharmacokinetics in cynomolgus monkeys and the safety profiles in rats and cynomolgus monkeys were assessed. DS-8201a exhibited a HER2 expression-dependent cell growth-inhibitory activity and induced tumor regression with a single dosing at more than 1 mg/kg in a HER2-positive gastric cancer NCI-N87 model. Binding activity to HER2 and ADCC activity of DS-8201a were comparable with unconjugated anti-HER2 antibody. DS-8201a also showed an inhibitory activity to Akt phosphorylation. DS-8201a induced phosphorylation of Chk1 and Histone H2A.X, the markers of DNA damage. Pharmacokinetics and safety profiles of DS-8201a were favorable and the highest non-severely toxic dose was 30 mg/kg in cynomolgus monkeys, supporting DS-8201a as being well tolerated in humans. DS-8201a was effective in a T-DM1-insensitive PDX model with high HER2 expression. DS-8201a, but not T-DM1, demonstrated antitumor efficacy against several breast cancer PDX models with low HER2 expression. DS-8201a exhibited a potent antitumor activity in a broad selection of HER2-positive models and favorable pharmacokinetics and safety profiles. The results demonstrate that DS-8201a will be a valuable therapy with a great potential to respond to T-DM1-insensitive HER2-positive cancers and low HER2-expressing cancers. Clin Cancer Res; 22(20); 5097-108. ©2016 AACR.

GMP-grade α-TEA lysine salt: a 28-Day oral toxicity and toxicokinetic study with a 28-Day recovery period in Beagle dogs.

BackgroundAlpha-tocopheryloxyacetic acid (α-TEA) is a semi-synthetic derivative of naturally occurring vitamin E (alpha-tocopherol) that can be delivered via an oral route. Preclinical in vitro and in vivo data demonstrated that α-TEA is a potent anti-tumor agent with a safe toxicity profile in mice. We report a comprehensive study to evaluate the toxokinetics of good manufacturing practice (GMP)-grade α-TEA in dogs after daily oral administration for 28 days, followed by a 28-day recovery period.MethodsMale and female beagle dogs received capsules of α-TEA Lysine Salt at doses of 100, 300, 1500 mg/kg/day. α-TEA plasma levels were determined by high-performance liquid chromatography (HPLC) with mass spectrometric detection. During the treatment, animals were observe for clinical signs, food consumption, body weight, and subjected to ophthalmoscopic, and electrocardiographic assessments. At the end of the dosing period, blood was taken and toxicokinetic analyses and histopathology assessments were performed when animals were necropsied.ResultsOur findings showed that there was no α-TEA-related mortality or moribundity. At the highest dose, increases in white blood cells and fibrinogen levels were observed. These levels returned to normal at the end of the recovery period. Histopathological evaluation of major organs revealed no significant lesions related to α-TEA-treatment.ConclusionWe demonstrate that for designing clinical trials in patients, the highest non-severely toxic dose (HNSTD) of α-TEA is 1500 mg/kg/day in Beagle dogs and this data informed the design of dose-escalation studies of α-TEA in patients with advanced cancer.

Choice of Starting Dose for Biopharmaceuticals in First-in-Human Phase I Cancer Clinical Trials.

First-in-human (FIH) trials of low-molecular-weight anticancer agents conventionally derive a safe start dose (SD) from one-tenth the severely toxic dose in 10% of rodents or one-sixth the highest nonseverely toxic dose (HNSTD) in nonrodent species. No consensus has been reached on whether this paradigm can be safely applied to biotechnology-derived products (BDPs). A comprehensive search was conducted to identify all BDPs (excluding immune checkpoint inhibitors and antibody drug conjugates) with sufficient nonclinical and clinical data to assess the safety of hypothetical use of one-sixth HNSTD in an advanced cancer FIH trial. The search identified 23 BDPs, of which 21 were monoclonal antibodies. The median ratio of the maximum tolerated or maximum administered dose (MTD or MAD) to the actual FIH SD was 36 (range, 8-500). Only 2 BDPs reached the MTD. Hypothetical use of one-sixth HNSTD (allometrically scaled to humans) would not have exceeded the MTD or MAD for all 23 BDPs and would have reduced the median ratio of the MTD or MAD to a SD to 6.1 (range, 3.5-55.3). Pharmacodynamic (PD) markers were included in some animal toxicology studies and were useful to confirm the hypothetical SD of one-sixth HNSTD. One-sixth HNSTD would not have resulted in unacceptable toxicities in the data available. Supporting its use could reduce the number of dose escalations needed to reach the recommended dose. A low incidence of toxicities in animals and humans underscores the need to identify the pharmacokinetic and PD parameters to guide SD selection of BDPs for FIH cancer trials. Start dose (SD) for biotechnology-derived products (BDPs) can be safely derived from one-sixth the highest nonseverely toxic dose in nonrodent species and may reduce the number of dose escalations needed to reach the recommended dose in first-in-human studies while limiting unnecessary exposure to high drug levels in humans. The use of this type of SD could improve the design of phase I studies of BDPs by making them more efficient. The role of preclinical pharmacodynamic markers was useful in confirming the hypothetical SD, and attempts should be explored in future animal studies to identify such parameters.

An FDA oncology analysis of antibody-drug conjugates

Antibody-drug conjugates (ADCs) are complex molecules composed of monoclonal antibodies conjugated to potent cytotoxic agents through chemical linkers. Because of this complexity, sponsors have used different approaches for the design of nonclinical studies to support the safety evaluation of ADCs and first-in-human (FIH) dose selection. We analyzed this data with the goal of describing the relationship between nonclinical study results and Phase 1 study outcomes. We summarized the following data from investigational new drug applications (INDs) for ADCs: plasma stability, animal study designs and toxicities, and algorithms used for FIH dose selection. Our review found that selecting a FIH dose that is 1/6th the highest non-severely toxic dose (HNSTD) in cynomolgus monkeys or 1/10th the STD10 in rodents scaled according to body surface area (BSA) generally resulted in the acceptable balance of safety and efficient dose-escalation in a Phase 1 trial. Other approaches may also be acceptable, e.g. 1/10th the HNSTD in monkeys using BSA or 1/10th the NOAEL in monkeys or rodents using body weight for scaling. While the animal data for the vc-MMAE platform yielded variable range of HNSTDs in cynomolgus monkeys, MTDs were in a narrow range in patients, suggesting that for ADCs sharing the same small molecule drug, linker and drug:antibody ratio, prior clinical data can inform the design of a Phase 1 clinical trial.

Abstract C91: Pharmacodynamic analysis of the Hsp90 inhibitor STA-9090 in a lung cancer xenograft model supports an infrequent dosing schedule in the clinic

Abstract Background: Heat shock protein 90 (Hsp90) is a molecular chaperone that is required for the stability and function of many important signal transduction proteins that regulate the growth of cancer cells. Hsp90 inhibition results in ubiquitination and proteasomal degradation of these client proteins, which include clinically validated drug targets such as BCR-ABL, mutant EGFR, HER2, KIT and VEGFR. STA-9090 is a novel small molecule Hsp90 inhibitor that is currently in multiple Phase 1/2 clinical trials in solid tumor and hematological malignancies. STA-9090 is structurally unrelated to the first-generation anasamycin Hsp90 inhibitors 17-AAG and IPI-504 and inhibits Hsp90 by binding to its N-terminal ATP-binding pocket. Although Hsp90 inhibitors such as STA-9090 induce rapid client protein degradation, cell cycle arrest and apoptosis of cancer cells, it is possible that frequent drug dosing in the clinic may be needed to continuously maintain decreased client protein expression and avoid renewed tumor growth. To investigate this possibility, we conducted in vitro and in vivo studies using the human NCI-H1975 non-small cell lung cancer cell line, which expresses the Hsp90 client protein EGFRL858R/T790M, a mutationally activated and erlotinib-resistant form of the epidermal growth factor receptor. Results: In an in vitro cytotoxicity assay using this cell line, STA-9090 and 17-AAG displayed IC50 values of 10 and 40 nM after 72 hr drug exposure, respectively. These results closely correlated with decreased expression of EGFRL858R/T790M and other Hsp90 client proteins. Unexpectedly, exposure to STA-9090 for only 1 hr still resulted in an IC50 of 670 nM, suggesting that even brief drug exposure in vivo may be sufficient to affect tumor growth. Consistent with this, intravenous dosing of 125 mg/kg STA-9090 on a 1X/week × 3 week schedule (∼80–100% of the highest non-severely toxic dose) induced stable disease in a NCI-H1975 xenograft model, whereas 175 mg/kg 17-AAG resulted in progressive disease, with %T/C values of 15 and 50, respectively. Inhibition of tumor growth was correlated with decreased expression of EGFRL858R/T790M and other client proteins, and importantly, these effects persisted in tumors for 3–6 days after a single drug dose. Similarly, histological analysis of tumors indicated that STA-9090 inhibited cell proliferation by 7-fold and induced apoptosis by 9-fold, with maximal effects being observed at 1–3 days after treatment. Consistent with these observations, STA-9090 accumulated in tumors relative to normal tissues, with a tumor half-life of 58 hr versus 3–5 hr in liver, lung and plasma, and the tumor concentration remained 140-fold higher than the in vitro IC50 (72 hr) even 6 days after a single drug dose. Conclusions: Taken together, these results demonstrate that STA-9090 is a highly potent Hsp90 inhibitor that selectively accumulates in tumors and induces long-lasting client protein degradation, cell cycle arrest, increased apoptosis and tumor growth inhibition in a lung cancer xenograft model. Our results suggest that an infrequent dosing schedule may have clinical activity in cancer patients. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C91.