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Address for correspondence: Maria Q. Baggstrom, MD, Alvin J. Siteman Cancer Center at Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110
AT-101 is an oral inhibitor of the antiapoptotic Bcl proteins (Bcl-2, Bcl-XL, Bcl-W, and Mcl-1) and an inducer of the pro-apoptotic proteins noxa and puma. We studied the efficacy of AT-101 in patients with recurrent chemosensitive extensive-stage small cell lung cancer (SCLC).
Methods
Patients with recurrent “sensitive” SCLC (defined as no progression during and no disease recurrence <2 months after completion of first-line platinum-based chemotherapy) were eligible. AT-101 was administered 20 mg orally daily for 21 of 28 days each cycle for up to six cycles. The primary end point was the objective response rate.
Results
At the time of planned interim evaluation, none of the 14 evaluable patients enrolled in the first stage had any response to therapy, and the study was closed permanently for further accrual. Three patients (21%) achieved stable disease after two cycles of therapy. Grade 3 toxicities included anorexia, fatigue, and nausea/vomiting.
Conclusions
AT-101 is not active in patients with recurrent chemosensitive SCLC.
Changing epidemiology of small-cell lung cancer in the United States over the last 30 years: analysis of the surveillance, epidemiologic, and end results database.
Although initial combination chemotherapy is very effective with response rates of 60 to 80%, the majority of patients will eventually die of recurrent disease. Although topotecan has been approved for use in patients with relapsed SCLC,
Clearly, new agents need to be developed to address this problem.
AT-101 (R-(−)-gossypol acetic acid) is an orally administered Bcl-2 homology (BH3) mimetic which has been reported to inhibit the heterodimerization of Bcl-xL, Bcl-w, and Mcl-1 with pro-apoptotic Bcl-2 proteins, thus lowering the threshold for apoptosis. As the racemic mixture, AT-101 has also been administered to more than 9000 healthy volunteer men as a male contraceptive.
It has been well tolerated with the most common adverse events being gastrointestinal toxicities and grade 1/2 fatigue. Other toxicities include cardiac effects including elevations of troponin levels and three possibly related cardiac rhythm abnormalities. Early studies have reported only limited single-agent antitumor activity.
Because AT-101 is a novel small molecule that inhibits the heterodimerization to the pro-apoptotic Bcl-2 family proteins, the administration of AT-101 could potentially be of value in patients with SCLC. Preclinical activity of AT-101 in SCLC, alone and in combination with standard chemotherapy agents, has been demonstrated using in vitro and in vivo models.
We, therefore, conducted a phase II study of single agent AT-101 in patients with chemotherapy-sensitive relapsed SCLC to characterize its activity.
PATIENTS AND METHODS
Eligible patients had histologically or cytologically confirmed SCLC with measurable disease per the Response Evaluation Criteria in Solid Tumors (RECIST)
New guidelines to evaluate the response to treatment in solid tumors. European Organization for Research and Treatment of Cancer, National Cancer Institute of the United States, National Cancer Institute of Canada.
and were previously treated with only one prior platinum-based chemotherapy regimen. They must have chemotherapy-sensitive disease defined as no progression during first-line chemotherapy and no disease recurrence sooner than 2 months after completion of first-line chemotherapy. Patients had an Eastern Cooperative Oncology Group performance status of 0 to 2 and a life expectancy of greater than 12 weeks. Required laboratory parameters included leukocytes ≥3,000/μL, absolute neutrophil count ≥1,500/μL, hemoglobin >8 g/dl, platelets ≥100,000/μL, total bilirubin <1.5 mg/dl, aspartate aminotransferase (SGOT) and alanine aminotransferase (SGPT) ≤2.5 × institutional upper limit of normal, and serum creatinine ≤1.5 mg/dl (or creatinine clearance ≥60 ml/min). All patients must be able to take oral medications on a regular basis and to give written informed consent. Patients with treated brain metastases who were clinically stable were eligible. The protocol was conducted by the Mayo Phase 2 Consortium along with the California Cancer Consortium and approved by the Institutional Review Boards of all participating institutions. All patients provided written informed consent before undergoing treatment.
AT-101 (NSC# 726190) was supplied by the National Cancer Institute Cancer Therapy Evaluation Program as tablets containing 10 mg of drug. AT-101 was self-administered at 20 mg orally once per day for 21 consecutive days followed by a 7-day rest. Treatment was repeated every 4 weeks for up to six cycles or until progressive disease, unacceptable toxicity, or withdrawal of consent. The dose of AT-101 was reduced to 10 mg once daily for prestudy-defined adverse event criteria. Patients who required more than one dose reduction due to toxicity were removed from the study.
As a potential biomarker of apoptotic pathway induction in response to AT-101, peripheral blood mononuclear cells (PBMC) were collected, where feasible, within 2 weeks before starting AT-101 and on day 2 of therapy. PBMC were isolated by low-speed centrifugation in CPT tubes, stored in OCT medium, and frozen at −80°C for subsequent analysis. Caspase activation in PBMC was assessed using the Caspase-Glo 3/7 Assay System (Promega) according to the manufacturer's instructions. Assays were performed in triplicate.
The primary end point of this trial was objective response rate, determined using RECIST. All patients who completed at least one postbaseline tumor assessment were evaluable for response. A two-stage Simon design
was used to test the null hypothesis (H0) that the true response probability is ≤0.10 versus the alternative hypothesis (H1) that the true response probability is ≥0.25. A total of 14 patients were to be accrued to the first stage. If two or more patients exhibited disease response, the study was to be continued to the second stage and an additional 28 patients were to be enrolled. If 7 or more patients of the 42 patients enrolled exhibited disease response, the regimen was to be considered worthy of further study in this population. This design has a probability (significance level alpha) of 0.10 to reject H0 when H0 is true and a probability of 0.85 to reject H0 when H1 is true (power). The secondary end points included toxicities, time to disease progression, and overall survival as well as intratumoral Bcl-2 family member expression and possible induction of the intrinsic apoptotic pathway. All toxicities were graded according the Common Terminology Criteria for Adverse Events version 3.0.
RESULTS
From November 2008 to January 2010, 15 patients were accrued to the study from five sites within the Mayo Phase 2 Consortium and the California Consortium. Patient characteristics are summarized in Table 1. The median number of cycles of AT-101 was 2 (range, 1–6). One patient discontinued therapy after one cycle of treatment due to persistent grade 1 thrombocytopenia which failed to recover to retreatment level of >100,000 per protocol after 3 weeks and was considered nonevaluable for the primary end point. Fourteen patients were evaluable for the primary end point at the interim analysis. Two patients required a dose delay due to a nonhematologic adverse event. One patient required dose modification due to a nonhematologic adverse event.
All grade 3/4 toxicities (regardless of attribution) are summarized in Table 2. Four patients experienced grade 3 adverse events attributable to the study treatment: nausea and vomiting (one patient), fatigue (one patient), and anorexia (two patients). Two patients experienced grade 3 hematologic adverse events (leukopenia), which were determined to be unrelated to treatment. No grade 4 toxicities were observed.
At the time of planned interim evaluation, among the 14 evaluable patients, no objective responses were observed. Therefore, accrual to the trial was terminated due to failure to pass the prespecified interim analysis per study design. Three patients (21%) achieved stable disease after two cycles but subsequently progressed on treatment. Median time to progression was 1.7 months (95% confidence interval: 1.5–2.0 months). Median overall survival was 8.5 months (95% confidence interval: 8.1–10.2 months) (FIGURE 1, FIGURE 2).
Caspase assays were performed on PBMC samples as a measure of the extent of activation of the apoptotic signaling cascade induced by AT-101 in vivo. Only three subjects had pre- and posttreatment samples available for analysis. There was marked interpatient variability in caspase activation and no consistent evidence of apoptotic induction by AT-101 (Figure 3). Only one of the three patients tested demonstrated evidence of increased caspase activity after AT-101 treatment, with two patients paradoxically showing lower caspase activity posttreatment. Each of the three patients had progressive disease as their best response.
FIGURE 3Relative caspase activity in peripheral blood mononuclear cells.
Unfortunately, AT-101 joins the long list of agents that have shown disappointing results in patients with SCLC. As a single agent in patients with chemosensitive recurrent SCLC in this trial, AT-101 did achieve stable disease in three patients after two cycles of treatment but those patients progressed while still on therapy. A phase I/II study of AT-101 in combination with topotecan in recurrent SCLC was recently published.
Heist RS, Fain J, Chinnasami B, et al. Phase I/II study of AT-101 with topotecan in relapsed and refractory small cell lung cancer. J Thorac Oncol 5:1637–1643.
The trial looked at two cohorts: chemorefractory and chemosensitive. The study did not meet its primary end points at the planned interim analysis and was stopped early.
Several hypotheses have been proposed as to the disappointing activity of Bcl-2 inhibitors in SCLC. To date, the promising preclinical findings with Bcl-2 inhibitors have failed to materialize in the clinic. A possible explanation is that the agents do not reach sufficient concentration in the tumor cells to suppress the Bcl-2 pathway in vivo. Another contributor to drug resistance could be the long half-life of Bcl-2 protein. A third explanation is the possibility of off-target cytotoxic mechanisms and a failure to inhibit apoptotic induction specifically. In a preclinical study of six Bcl-2 inhibitors, both obatoclax and AT-101 treatment caused cell death in a caspase-independent manner instead of inducing classic apoptosis.
Only ABT-737 was noted to induce classic apoptosis. This failure to induce caspase-dependent cell death is in keeping with our limited correlative analyses, which demonstrated no consistent induction on caspases after AT-101 treatment. One recent study identified some gene expression signatures that could predict sensitivity to ABT-263 in both SCLC and leukemia/lymphoma lines.
Tahir SK, Wass J, Joseph MK, et al. Identification of expression signatures predictive of sensitivity to the Bcl-2 family member inhibitor ABT-263 in small cell lung carcinoma and leukemia/lymphoma cell lines. Mol Cancer Ther 9:545–557.
Perhaps, the combination of the current Bcl-2 inhibitors with agents that target other areas of the apoptotic pathway may cause adequate suppression. Another avenue would be to further explore gene expression signatures as predictors of tumor sensitivity to target subsets of the patient population that would be sensitive to Bcl-2 inhibition.
ACKNOWLEDGMENTS
Supported by N01-CM62205 and in part by Cancer Center Support Grant P30 CA91842 and the Clinical Trials Core of the Siteman Cancer Center at Washington University and Barnes-Jewish Hospital in St. Louis, MO.
REFERENCES
Govindan R
Page N
Morgensztern D
et al.
Changing epidemiology of small-cell lung cancer in the United States over the last 30 years: analysis of the surveillance, epidemiologic, and end results database.
New guidelines to evaluate the response to treatment in solid tumors. European Organization for Research and Treatment of Cancer, National Cancer Institute of the United States, National Cancer Institute of Canada.
Heist RS, Fain J, Chinnasami B, et al. Phase I/II study of AT-101 with topotecan in relapsed and refractory small cell lung cancer. J Thorac Oncol 5:1637–1643.
Tahir SK, Wass J, Joseph MK, et al. Identification of expression signatures predictive of sensitivity to the Bcl-2 family member inhibitor ABT-263 in small cell lung carcinoma and leukemia/lymphoma cell lines. Mol Cancer Ther 9:545–557.
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