Gossypolone

AT101, a natural product from cottonseed with a BH3-mimetic structure, was identified as a small molecule inhibitor of Bcl-2/Bcl-xL/Mcl-1 that potently induces apoptosis in various cancer cell lines [1].It is one of the world's first small molecule Bcl-2 inhibitors that has entered into clinical trials and is now in phase II clinical trials for hormone-refractory prostate cancer and other types of cancers [2, 3]. Few side effects of gossypol have been reported, with the major side effects of gossypol being nausea and vomiting in the third month of treatment or rashes earlier in the course of treatment [4]. Thus, AT101 is clinically safe [2, 3] and could be used as a potential inducer of apoptosis in cancer treatment.
As a BH3 mimetic, AT-101 binds to the hydrophobic surface binding groove BH3 of the anti-apoptotic proteins Bcl-2 and Bcl-xL, blocking their heterodimerization with pro-apoptotic members of the Bcl-2 family of proteins such as Bad, Bid, and Bim; this may result in the inhibition of tumor cell proliferation and the induction of tumor cell apoptosis. Preclinical studies revealed that gossypol not only interrupts the interaction between anti- and proapoptotic Bcl-2 family proteins but also induces BH3 protein (such as Puma and Noxa) up-regulation or down-regulates XIAP expression [5]. Thus, gossypol can induce apoptosis by activating apoptogenic factors other than the Bcl-2 family. AT-101 induced apoptosis in vitro through activation of caspase-9.
AT101 delayed onset of androgen-independent growth of VCaP prostate cancer xenografts in vivo. Gossypol can neutralize antiapoptotic Bcl-2 proteins and induced Bax activation [1]. However, the function of gossypol was not limited to effects on the interaction between anti- and proapoptotic Bcl-2 proteins. Some studies have demonstrated that gossypol could down-regulate Bcl-2, Bcl-xL, and XIAP expression [6] or induce Puma and Noxa expression. Therefore, the apoptotic effect of gossypol has been demonstrated to be attenuated by the presence of androgen in a prostate cancer xenograft mouse model.
References:
[1]. Meng Y, Tang W,Dai Y, et al. Natural BH3-mimetic (-)-gossypol chemosensitizes human prostate cancer via Bcl-xL inhibition accompanied by increase of Puma and Noxa.Molecular cancer therapeutics,2008,7(7): 2192–2202.
[2]. Liu G,Kelly W. K,Wilding G,et al. An Open-Label, Multicenter, Phase I/II Study of Single-Agent AT-101 in Men with Castrate-Resistant Prostate Cancer (CRPC).Clinical Cancer Research,2009,15(9): 3172–3176.
[3]. Van Poznak C, Seidman A. D,Reidenberg M,et al. Oral gossypol in the treatment of patients with refractory metastatic breast cancer: a phase I/II clinical trial,Breast Cancer ResTreat ,2001,66(3):239-48.
[4]. Qiu J,Levin L. R,Buck J,et al. Different pathways of cell killing by gossypol enantiomers.Exp Biol Med (Maywood) ,2002,227(6):398-401.
[5]. Balakrishnan K,Burger J. A,Wierda W. G,et al. AT-101 induces apoptosis in CLL B cells and overcomes stromal cell–mediated Mcl-1 induction and drug resistance.Blood,2009,113(1): 149–153.
[6].Sung B,Ravindran J,Prasad S,et al.Gossypol Induces Death Receptor-5 through Activation of the ROS-ERK-CHOP Pathway and Sensitizes Colon Cancer Cells to TRAIL.J Biol Chem,2010,285(46): 35418–35427.

A phase II trial of the BCL-2 homolog domain 3 mimetic AT-101 in combination with docetaxel for recurrent, locally advanced, or metastatic head and neck cancer.[Pubmed:27225873]

Invest New Drugs. 2016 Aug;34(4):481-9.

BACKGROUND: AT-101 is a BCL-2 Homolog domain 3 mimetic previously demonstrated to have tumoricidal effects in advanced solid organ malignancies. Given the evidence of activity in xenograft models, treatment with AT-101 in combination with docetaxel is a therapeutic doublet of interest in metastatic head and neck squamous cell carcinoma. PATIENTS AND METHODS: Patients included in this trial had unresectable, recurrent, or distantly metastatic head and neck squamous cell carcinoma (R/M HNSCC) not amenable to curative radiation or surgery. This was an open label randomized, phase II trial in which patients were administered AT-101 in addition to docetaxel. The three treatment arms were docetaxel, docetaxel plus pulse dose AT-101, and docetaxel plus metronomic dose AT-101. The primary endpoint of this trial was overall response rate. RESULTS: Thirty-five patients were registered and 32 were evaluable for treatment response. Doublet therapy with AT-101 and docetaxel was well tolerated with only 2 patients discontinuing therapy due to treatment related toxicities. The overall response rate was 11 % (4 partial responses) with a clinical benefit rate of 74 %. Median progression free survival was 4.3 months (range: 0.7-13.7) and overall survival was 5.5 months (range: 0.4-24). No significant differences were noted between dosing strategies. CONCLUSION: Although met with a favorable toxicity profile, the addition of AT-101 to docetaxel in R/M HNSCC does not appear to demonstrate evidence of efficacy.

AT-101 enhances gefitinib sensitivity in non-small cell lung cancer with EGFR T790M mutations.[Pubmed:27431492]

BMC Cancer. 2016 Jul 18;16:491.

BACKGROUND: Although epidermal growth factor receptor tyrosine kinase inhibitors (EGFR TKIs) have become the standard care of patients with advanced EGFR-mutant non-small cell lung cancer (NSCLC), development of acquired resistance is inevitable. A secondary mutation of threonine 790 (T790M) is associated with approximately half of the cases of acquired resistance. Strategies or agents to overcome this type of resistance are still limited. In this study, enhanced antitumor effect of AT-101, a-pan-Bcl-2 inhibitor, on gefitinib was explored in NSCLC with T790M mutation. METHODS: The effect of cotreatment with AT-101 and gefitinib on the viability of NSCLC cell lines harboring acquired T790M mutation was investigated using the MTT assay. The cellular apoptosis of NSCLC cells after cotreatment with AT-101 and gefitinib was assessed by FITC-annexin V/PI assay and Western blots analysis. The potential underlying mechanisms of the enhanced therapeutic effect for AT-101 was also studied using Western blots analysis. The in vivo anti-cancer efficacy of the combination with AT-101 and gefitinib was examined in a mouse xenograft model. RESULTS: In this study, we found that treatment with AT-101 in combination with gefitinib significantly inhibited cell proliferation, as well as promoted apoptosis of EGFR TKIs resistant lung cancer cells. The apoptotic effects of the use of AT-101 was related to the blocking of antiapoptotic protein: Bcl-2, Bcl-xl, and Mcl-1 and downregrulation of the molecules in EGFR pathway. The observed enhancements of tumor growth suppression in xenografts supported the reverse effect of AT-101 in NSCLC with T790M mutation, which has been found in in vitro studies before. CONCLUSIONS: AT-101 enhances gefitinib sensitivity in NSCLC with EGFR T790M mutations. The addition of AT-101 to gefitinib is a promising strategy to overcome EGFR TKIs resistance in NSCLC with EGFR T790M mutations.

AT-101 simultaneously triggers apoptosis and a cytoprotective type of autophagy irrespective of expression levels and the subcellular localization of Bcl-xL and Bcl-2 in MCF7 cells.[Pubmed:26721623]

Biochim Biophys Acta. 2016 Apr;1863(4):499-509.

The effects of autophagy on cell death are highly contextual and either beneficial or deleterious. One prime example for this dual function of autophagy is evidenced by the cell responses to the BH3 mimetic AT-101 that is known to induce either apoptotic or autophagy-dependent cell death in different settings. Based on previous reports, we hypothesized that the expression levels of pro-survival Bcl-2 family members may be key determinants for the respective death mode induced by AT-101. Here we investigated the role of autophagy in the response of MCF7 breast cancer cells to AT-101. AT-101 treatment induced a prominent conversion of LC3-I to LC3-II and apoptotic cell death characterized by the appearance of Annexin-positive/PI-negative early apoptotic cells and PARP cleavage. Inhibition of the autophagy pathway, either through application of 3-MA or by lentiviral knockdown of ATG5, strongly potentiated cell death, indicating a pro-survival function of autophagy. Overexpression of wild type Bcl-xL significantly diminished the net amount of AT-101-induced cell death, but failed to alter the death-enhancing effects of the ATG5 knockdown. This was also observed with the organelle-specific variants Bcl-xL-ActA and Bcl-2-ActA (mitochondrial) as well as Bcl-xL-cb5 and Bcl-2-cb5 (ER) which all reduced AT-101-induced cell death, but did not affect the death-enhancing effects of 3-MA. Collectively, our data indicate that in apoptosis-proficient MCF7 cells, AT-101 triggers Bcl-2- and Bcl-xL-dependent apoptosis and a cytoprotective autophagy response that is independent of the expression and subcellular localization of Bcl-xL and Bcl-2.