Novel 6-aminofuro[3,2-c]pyridines as potent, orally efficacious inhibitors of cMET and RON kinases.

A series of novel 6-aminofuro[3,2-c]pyridines as kinase inhibitors is described, most notably, OSI-296 (6). We discuss our exploration of structure-activity relationships and optimization leading to OSI-296 and disclose its pharmacological activity against cMET and RON in cellular assays. OSI-296 is a potent and selective inhibitor of cMET and RON kinases that shows in vivo efficacy in tumor xenografts models upon oral dosing and is well tolerated.

Cooperative signaling between oncostatin M, hepatocyte growth factor and transforming growth factor-ß enhances epithelial to mesenchymal transition in lung and pancreatic tumor models.

Epithelial to mesenchymal transition (EMT) plays a dual role in tumor progression. It enhances metastasis of tumor cells by increasing invasive capacity and promoting survival, and it decreases tumor cell sensitivity to epithelial cell-targeting agents such as epithelial growth factor receptor kinase inhibitors. In order to study EMT in tumor cells, we have characterized 3 new models of ligand-driven EMT: the CFPAC1 pancreatic tumor model and the H358 and H1650 lung tumor models. We identified a diverse set of ligands that drives EMT in these models. Hepatocyte growth factor and oncostatin M induced EMT in all models, while transforming growth factor-ß induced EMT in both lung models. We observed morphologic, marker and phenotypic changes in response to chronic ligand treatment. Interestingly, stimulation with 2 ligands resulted in more pronounced EMT compared with single-ligand treatment, demonstrating a spectrum of EMT states induced by parallel signaling, such as the JAK and PI3K pathways. The EMT changes observed in response to the ligand were reversed upon ligand withdrawal, demonstrating the 'metastable' nature of these models. To study the impact of EMT on cell morphology and invasion in a 3D setting, we cultured cells in a semisolid basement membrane extract. Upon stimulation with EMT ligands, the colonies exhibited changes to EMT markers and showed phenotypes ranging from modest differences in colony architecture (CFPAC1) to complex branching structures (H358, H1650). Collectively, these 3 models offer robust cell systems with which to study the roles that EMT plays in cancer progression.

Evaluation of the dual mTOR/PI3K inhibitors Gedatolisib (PF-05212384) and PF-04691502 against ovarian cancer xenograft models.

This study investigated the antitumour effects of two dual mTOR/PI3K inhibitors, gedatolisib (WYE-129587/PKI-587/PF-05212384) and PF-04691502 against a panel of six human patient derived ovarian cancer xenograft models. Both dual mTOR/PI3K inhibitors demonstrated antitumour activity against all xenografts tested. The compounds produced tumour stasis during the treatment period and upon cessation of treatment, tumours re-grew. In several models, there was an initial rapid reduction of tumour volume over the first week of treatment before tumour stasis. No toxicity was observed during treatment. Biomarker studies were conducted in two xenograft models; phospho-S6 (Ser235/236) expression (as a readout of mTOR activity) was reduced over the treatment period in the responding xenograft but expression increased to control (no treatment) levels on cessation of treatment. Phospho-AKT (Ser473) expression (as a readout of PI3K) was inhibited by both drugs but less markedly so than phospho-S6 expression. Initial tumour volume reduction on treatment and regrowth rate after treatment cessation was associated with phospho-S6/total S6 expression ratio. Both drugs produced apoptosis but minimally influenced markers of proliferation (Ki67, phospho-histone H3). These results indicate that mTOR/PI3K inhibition can produce broad spectrum tumour growth stasis in ovarian cancer xenograft models during continuous chronic treatment and this is associated with apoptosis.

Colon carcinoma cells harboring PIK3CA mutations display resistance to growth factor deprivation induced apoptosis.

PIK3CA, encoding the p110alpha catalytic subunit of phosphatidylinositol 3-kinase (PI3K), is mutated in a variety of human cancers. We screened the colon cancer cell lines previously established in our laboratory for PIK3CA mutations and found that four of them harbored gain of function mutations. We have now compared a panel of mutant and wild-type cell lines for cell proliferation and survival in response to stress. There was little difference in PI3K activity between mutant PIK3CA-bearing cells (mutant cells) and wild-type PIK3CA-bearing cells (wild-type cells) under optimal growth conditions. However, the mutant cells showed constitutive PI3K activity during growth factor deprivation stress (GFDS), whereas PI3K activity decayed rapidly in the wild-type cells. Importantly, constitutively active PI3K rendered the mutant cells resistant to GFDS-induced apoptosis relative to the wild-type cells, indicating a biological advantage under stress conditions that is imparted by the mutant enzymes. Compared with the wild-type cells, the mutant cells were hypersensitive to the apoptosis induced by the PI3K inhibitor LY294002. In addition, PIK3CA small interfering RNA significantly decreased DNA synthesis and/or induced apoptosis in the mutant cells but not in the wild-type cells. Furthermore, ecotopic expression of a mutant PIK3CA in a nontumorigenic PIK3CA wild-type cell line resulted in resistance to GFDS-induced apoptosis, whereas transfection of wild-type PIK3CA or empty vector had little effect. Taken together, our studies show that mutant PIK3CA increases the capacity for proliferation and survival under environmental stresses, such as GFDS while also imparting greater dependency on the PI3K pathway for proliferation and survival.

Resistance to dual blockade of the kinases PI3K and mTOR in KRAS-mutant colorectal cancer models results in combined sensitivity to inhibition of the receptor tyrosine kinase EGFR.

Targeted blockade of aberrantly activated signaling pathways is an attractive therapeutic strategy for solid tumors, but drug resistance is common. KRAS is a frequently mutated gene in human cancer but remains a challenging clinical target. Inhibitors against KRAS signaling mediators, namely, PI3K (phosphatidylinositol 3-kinase) and mTOR (mechanistic target of rapamycin), have limited clinical efficacy as single agents in KRAS-mutant colorectal cancer (CRC). We investigated potential bypass mechanisms to PI3K/mTOR inhibition in KRAS-mutant CRC. Using genetically engineered mouse model cells that had acquired resistance to the dual PI3K/mTOR small-molecule inhibitor PF-04691502, we determined with chemical library screens that inhibitors of the ERBB [epidermal growth factor receptor (EGFR)] family restored the sensitivity to PF-04691502. Although EGFR inhibitors alone have limited efficacy in reducing KRAS-mutant tumors, we found that PF-04691502 induced the abundance, phosphorylation, and activity of EGFR, ERBB2, and ERBB3 through activation of FOXO3a (forkhead box O 3a), a transcription factor inhibited by the PI3K to AKT pathway. PF-04691502 also induced a stem cell-like gene expression signature. KRAS-mutant patient-derived xenografts from mice treated with PF-04691502 had a similar gene expression signature and exhibited increased EGFR activation, suggesting that this drug-induced resistance mechanism may occur in patients. Combination therapy with dacomitinib (a pan-ERBB inhibitor) restored sensitivity to PF-04691502 in drug-resistant cells in culture and induced tumor regression in drug-resistant allografts in mice. Our findings suggest that combining PI3K/mTOR and EGFR inhibitors may improve therapeutic outcome in patients with KRAS-mutant CRC.

Tumor P-Glycoprotein Correlates with Efficacy of PF-3758309 in in vitro and in vivo Models of Colorectal Cancer.

P-glycoprotein (P-gp), a member of the ATP-binding cassette transporter family, is overexpressed in a number of different cancers and some studies show that P-gp overexpression can be correlated to poor prognosis or therapeutic resistance. Here we sought to elucidate if PF-3758309 (PF-309), a novel p-21 activated kinase inhibitor, efficacy was influenced by tumor P-gp. Based on in vitro proliferation data, a panel of colorectal cancer cell lines were ranked as sensitive or resistant and ABCB1 (P-gp) expression was evaluated by microarray for these cell lines. P-gp expression was determined by western blot and activity determined by rhodamine efflux assay. Knock down of P-gp and pharmacologic inhibition of P-gp to restore PF-309 activity was performed in vitro. PF-309 activity was evaluated in vivo in cell line xenograft models and in primary patient derived tumor xenografts (PDTX). Mice were treated with 25 mg/kg PF-309 orally, twice daily. On the last day of treatment, tumor and plasma were collected for PF-309 analysis. Here we show that ABCB1 gene expression correlates with resistance to PF-309 treatment in vitro and the expression and activity of P-gp was verified in a panel of resistant cells. Furthermore, inhibition of P-gp increased the sensitivity of resistant cells, resulting in a 4-100-fold decrease in the IC50s. Eleven cell line xenografts and 12 PDTX models were treated with PF-309. From the cell line xenografts, we found a significant correlation between ABCB1 gene expression profiles and tumor response. We evaluated tumor and plasma concentrations for eight tumor models (three cell line xenografts and five PDTX models) and a significant correlation was found between tumor concentration and response. Additionally, we show that tumor concentration is approximately fourfold lower in tumors that express P-gp, verified by western blot. Our in vitro and in vivo data strongly suggests that PF-309 efficacy is influenced by the expression of tumor P-gp.

Molecular predictors of sensitivity to the insulin-like growth factor 1 receptor inhibitor Figitumumab (CP-751,871).

Figitumumab (CP-751,871), a potent and fully human monoclonal anti-insulin-like growth factor 1 receptor (IGF1R) antibody, has been investigated in clinical trials of several solid tumors. To identify biomarkers of sensitivity and resistance to figitumumab, its in vitro antiproliferative activity was analyzed in a panel of 93 cancer cell lines by combining in vitro screens with extensive molecular profiling of genomic aberrations. Overall response was bimodal and the majority of cell lines were resistant to figitumumab. Nine of 15 sensitive cell lines were derived from colon cancers. Correlations between genomic characteristics of cancer cell lines with figitumumab antiproliferative activity revealed that components of the IGF pathway, including IRS2 (insulin receptor substrate 2) and IGFBP5 (IGF-binding protein 5), played a pivotal role in determining the sensitivity of tumors to single-agent figitumumab. Tissue-specific differences among the top predictive genes highlight the need for tumor-specific patient selection strategies. For the first time, we report that alteration or expression of the MYB oncogene is associated with sensitivity to IGF1R inhibitors. MYB is dysregulated in hematologic and epithelial tumors, and IGF1R inhibition may represent a novel therapeutic opportunity. Although growth inhibitory activity with single-agent figitumumab was relatively rare, nine combinations comprising figitumumab plus chemotherapeutic agents or other targeted agents exhibited properties of synergy. Inhibitors of the ERBB family were frequently synergistic and potential biomarkers of drug synergy were identified. Several biomarkers of antiproliferative activity of figitumumab both alone and in combination with other therapies may inform the design of clinical trials evaluating IGF1R inhibitors.

Development of a colon cancer GEMM-derived orthotopic transplant model for drug discovery and validation.

PURPOSE: Effective therapies for KRAS-mutant colorectal cancer (CRC) are a critical unmet clinical need. Previously, we described genetically engineered mouse models (GEMM) for sporadic Kras-mutant and non-mutant CRC suitable for preclinical evaluation of experimental therapeutics. To accelerate drug discovery and validation, we sought to derive low-passage cell lines from GEMM Kras-mutant and wild-type tumors for in vitro screening and transplantation into the native colonic environment of immunocompetent mice for in vivo validation. EXPERIMENTAL DESIGN: Cell lines were derived from Kras-mutant and non-mutant GEMM tumors under defined media conditions. Growth kinetics, phosphoproteomes, transcriptomes, drug sensitivity, and metabolism were examined. Cell lines were implanted in mice and monitored for in vivo tumor analysis. RESULTS: Kras-mutant cell lines displayed increased proliferation, mitogen-activated protein kinase signaling, and phosphoinositide-3 kinase signaling. Microarray analysis identified significant overlap with human CRC-related gene signatures, including KRAS-mutant and metastatic CRC. Further analyses revealed enrichment for numerous disease-relevant biologic pathways, including glucose metabolism. Functional assessment in vitro and in vivo validated this finding and highlighted the dependence of Kras-mutant CRC on oncogenic signaling and on aerobic glycolysis. CONCLUSIONS: We have successfully characterized a novel GEMM-derived orthotopic transplant model of human KRAS-mutant CRC. This approach combines in vitro screening capability using low-passage cell lines that recapitulate human CRC and potential for rapid in vivo validation using cell line-derived tumors that develop in the colonic microenvironment of immunocompetent animals. Taken together, this platform is a clear advancement in preclinical CRC models for comprehensive drug discovery and validation efforts.

Evaluation of the antitumor effects and mechanisms of PF00299804, a pan-HER inhibitor, alone or in combination with chemotherapy or targeted agents in gastric cancer.

Recently, HER2-directed treatment, such as trastuzumab, has shown clinical benefit in HER2-amplified gastric cancer. On the basis of recent studies about epidermal growth factor receptor (EGFR) or HER2-targeting agents (including gefitinib, lapatinib, and trastuzumab) in gastric cancer, the potent effects of pan-HER inhibitors targeting the HER family are anticipated. In this study, we evaluated the activity and mechanisms of PF00299804, an irreversible pan-HER inhibitor, in gastric cancer in vitro and in vivo models. PF00299804 showed significant growth-inhibitory effects in HER2-amplified gastric cancer cells (SNU216, N87), and it had lower 50% inhibitory concentration values compared with other EGFR tyrosine kinase inhibitors, including gefitinib, lapatinib, BIBW-2992, and CI-1033. PF00299804 induced apoptosis and G(1) arrest and inhibited phosphorylation of receptors in the HER family and downstream signaling pathways including STAT3, AKT, and extracellular signal-regulated kinases (ERK) in HER2-amplified gastric cancer cells. PF00299804 also blocked EGFR/HER2, HER2/HER3, and HER3/HER4 heterodimer formation as well as the association of HER3 with p85α in SNU216 cells. The combination of PF00299804 with clinically relevant chemotherapeutic agents or molecular-targeted agents including trastuzumab (an anti-HER2 monoclonal antibody), CP751871 (an IGF1R inhibitor), PD0325901 (an ERK1/2 inhibitor), and PF04691502 (a PI3K/mTOR inhibitor) produced synergistic effects. These findings indicate that PF00299804 can be used as a targeted therapy for the treatment of HER2-amplified gastric cancer through inhibition of HER family heterodimer formation and may augment antitumor efficacy of chemotherapeutic and/or molecular-targeted agents.

Combination of a MEK inhibitor at sub-MTD with a PI3K/mTOR inhibitor significantly suppresses growth of lung adenocarcinoma tumors in Kras(G12D-LSL) mice.

The role of PI3K and MAPK pathways in tumor initiation and progression is well established; hence, several inhibitors of these pathways are currently in different stages of clinical trials. Recent studies identified a PI3K/mTOR (PF-04691502) and a MEK inhibitor (PD-0325901) with strong potency and efficacy in different cell lines and tumor models. PD-0325901, however, showed adverse effects when administered at or above MTD (maximum tolerated dose) in the clinic. Here, we show in preclinical models that PD-0325901 at doses well below MTD (sub-MTD 1.5 mg/kg SID) is still a potent compound as single agent or in combination with PF-04691502. We first observed that PD-0325901 at 1.5 mg/kg SID and in combination with PF-04691502 (7.5 mg/kg; SID) significantly inhibited growth of H460 (carry Kras and PIK3CA mutations) orthotopic lung tumors. Additionally, we tested efficacy of PD-0325901 in Kras(G12D-LSL) conditional GEMMs (genetically engineered mouse models) which are a valuable tool in translational research to study tumor progression. Intranasal delivery of adenoviruses expressing Cre recombinase (Adeno-Cre) resulted in expression of mutant Kras leading to development of tumor lesions in lungs including adenomatous hyperplasia, large adenoma, and adenocarcinoma. Similar to H460 tumors, PD-0325901 as single agent or in combination with PF-04691502 significantly inhibited growth of tumor lesions in lungs in Kras(G12D-LSL) mice when treatment started at adenocarcinoma stage (at 14 weeks post-Adeno-Cre inhalation). In addition, immunohistochemistry showed inhibition of pS6 (phosphorylated ribosomal S6) in the treated animals particularly in the combination group providing a proof of mechanism for tumor growth inhibition. Finally, m-CT imaging in live Kras(G12D-LSL) mice showed reduction of tumor burdens in PD-0325901-treated animals at sub-MTD dose. In conclusion, our data suggest that PD-0325901 at doses below MTD is still a potent compound capable of tumor growth inhibition where Kras and/or PI3K are drivers of tumor growth and progression.

An integrated genomic approach to identify predictive biomarkers of response to the aurora kinase inhibitor PF-03814735.

PF-03814735 is a novel, reversible inhibitor of Aurora kinases A and B that finished a phase I clinical trial for the treatment of advanced solid tumors. To find predictive biomarkers of drug sensitivity, we screened a diverse panel of 87 cancer cell lines for growth inhibition upon PF-03814735 treatment. Small cell lung cancer (SCLC) and, to a lesser extent, colon cancer lines were very sensitive to PF-03814735. The status of the Myc gene family and retinoblastoma pathway members significantly correlated with the efficacy of PF-03814735. Whereas RB1 inactivation, intact CDKN2A/p16, and normal CCND1/Cyclin D1 status are hallmarks of SCLC, activation or amplification of any of the three Myc genes (MYC, MYCL1, and MYCN) clearly differentiated cell line sensitivity within the SCLC panel. By contrast, we found that expression of Aurora A and B were weak predictors of response. We observed a decrease in histone H3 phosphorylation and polyploidization of sensitive lines, consistent with the phenotype of Aurora B inhibition. In vivo experiments with two SCLC xenograft models confirmed the sensitivity of Myc gene-driven models to PF-03814735 and a possible schedule dependence of MYC/c-Myc-driven tumors. Altogether our results suggest that SCLC and other malignancies driven by the Myc family genes may be suitable indications for treatment by Aurora B kinase inhibitors.

Inducible expression of TGFß, snail and Zeb1 recapitulates EMT in vitro and in vivo in a NSCLC model.

The progression of cancer from non-metastatic to metastatic is the critical transition in the course of the disease. The epithelial to mesenchymal transition (EMT) is a mechanism by which tumor cells acquire characteristics that improve metastatic efficiency. Targeting EMT processes in patients is therefore a potential strategy to block the transition to metastatic cancer and improve patient outcome. To develop models of EMT applicable to in vitro and in vivo settings, we engineered NCI-H358 non-small cell lung carcinoma cells to inducibly express three well-established drivers of EMT: activated transforming growth factor ß (aTGFß), Snail or Zeb1. We characterized the morphological, molecular and phenotypic changes induced by each of the drivers and compared the different end-states of EMT between the models. Both in vitro and in vivo, induction of the transgenes Snail and Zeb1 resulted in downregulation of epithelial markers and upregulation of mesenchymal markers, and reduced the ability of the cells to proliferate. Induced autocrine expression of aTGFß caused marker and phenotypic changes consistent with EMT, a modest effect on growth rate, and a shift to a more invasive phenotype. In vivo, this manifested as tumor cell infiltration of the surrounding mouse stromal tissue. Overall, Snail and Zeb1 were sufficient to induce EMT in the cells, but aTGFß induced a more complex EMT, in which changes in extracellular matrix remodeling components were pronounced.

PF-04691502, a potent and selective oral inhibitor of PI3K and mTOR kinases with antitumor activity.

Deregulation of the phosphoinositide 3-kinase (PI3K) signaling pathway such as by PTEN loss or PIK3CA mutation occurs frequently in human cancer and contributes to resistance to antitumor therapies. Inhibition of key signaling proteins in the pathway therefore represents a valuable targeting strategy for diverse cancers. PF-04691502 is an ATP-competitive PI3K/mTOR dual inhibitor, which potently inhibited recombinant class I PI3K and mTOR in biochemical assays and suppressed transformation of avian fibroblasts mediated by wild-type PI3K γ, δ, or mutant PI3Kα. In PIK3CA-mutant and PTEN-deleted cancer cell lines, PF-04691502 reduced phosphorylation of AKT T308 and AKT S473 (IC(50) of 7.5-47 nmol/L and 3.8-20 nmol/L, respectively) and inhibited cell proliferation (IC(50) of 179-313 nmol/L). PF-04691502 inhibited mTORC1 activity in cells as measured by PI3K-independent nutrient stimulated assay, with an IC(50) of 32 nmol/L and inhibited the activation of PI3K and mTOR downstream effectors including AKT, FKHRL1, PRAS40, p70S6K, 4EBP1, and S6RP. Short-term exposure to PF-04691502 predominantly inhibited PI3K, whereas mTOR inhibition persisted for 24 to 48 hours. PF-04691502 induced cell cycle G(1) arrest, concomitant with upregulation of p27 Kip1 and reduction of Rb. Antitumor activity was observed in U87 (PTEN null), SKOV3 (PIK3CA mutation), and gefitinib- and erlotinib-resistant non-small cell lung carcinoma xenografts. In summary, PF-04691502 is a potent dual PI3K/mTOR inhibitor with broad antitumor activity. PF-04691502 has entered phase I clinical trials.

Knockdown of Ron kinase inhibits mutant phosphatidylinositol 3-kinase and reduces metastasis in human colon carcinoma.

Abnormal accumulation and activation of receptor tyrosine kinase Ron (recepteur d'origine nantais) has been demonstrated in a variety of primary human cancers. We show that RNA interference-mediated knockdown of Ron kinase in a highly tumorigenic colon cancer cell line led to reduced proliferation as compared with the control cells. Decreased Ron expression sensitized HCT116 cells to growth factor deprivation stress-induced apoptosis as reflected by increased DNA fragmentation and caspase 3 activation. In addition, cell motility was decreased in Ron knockdown cells as measured by wound healing assays and transwell assays. HCT116 cells are heterozygous for gain of function mutant PIK3CA H1047R. Analysis of signaling proteins that are affected by Ron knockdown revealed that phosphatidylinositol 3-kinase (PI3K) activity of the mutant PI3K as well as AKT phosphorylation was substantially reduced in the Ron knockdown cells compared with the control cells. Moreover, we demonstrated in vivo that knockdown of Ron expression significantly reduced lung metastasis as compared with the control cells in the orthotopic models. In summary, our results demonstrate that Ron plays an essential role in maintaining malignant phenotypes of colon cancer cells through regulating mutant PI3K activity. Therefore, targeting Ron kinase could be a potential strategy for colon cancer treatment, especially in patients bearing gain of function mutant PI3K activity.

3-[6-(2-Dimethylamino-1-imidazol-1-yl-butyl)-naphthalen-2-yloxy]-2,2-dimethyl-propionic acid as a highly potent and selective retinoic acid metabolic blocking agent.

3-[6-(2-Dimethylamino-1-imidazol-1-yl-butyl)-naphthalen-2-yloxy]-2,2-dimethyl-propionic acid and analogs were designed and synthesized as highly potent and selective CYP26 inhibitors, serving as retinoic acid metabolic blocking agents (RAMBAs), with demonstrated in vivo efficacy to increase the half-life of exogenous atRA.

Potent and selective [2-imidazol-1-yl-2-(6-alkoxy-naphthalen-2-yl)-1-methyl-ethyl]-dimethyl-amines as retinoic acid metabolic blocking agents (RAMBAs).

A series of [2-imidazol-1-yl-2-(6-alkoxy-naphthalen-2-yl)-1-methyl-ethyl]-dimethyl-amines were designed and synthesized as CYP26 inhibitors, serving as retinoic acid metabolic blocking agents (RAMBA's).

Arginine-serine-rich domains bound at splicing enhancers contact the branchpoint to promote prespliceosome assembly.

Exonic splicing enhancers (ESEs) are required for splicing of certain pre-mRNAs and function by providing binding sites for serine-arginine (SR) proteins, which contain an arginine-serine-rich (RS) domain. How an RS domain bound at the ESE promotes splicing is poorly understood. We have developed an RNA-protein crosslinking procedure to identify the target of the ESE-bound RS domain. Using this approach, we show that the ESE-bound RS domain specifically contacts the pre-mRNA branchpoint. The interaction between the ESE-bound RS domain and the branchpoint occurs in the prespliceosome and is dependent upon the same splicing signals, biochemical factors, and reaction conditions required to support prespliceosome assembly. Analysis of RS domain mutants demonstrates that the ability to interact with the branchpoint, to promote prespliceosome assembly, and to support splicing are related activities. We conclude that the ESE-bound RS domain functions by contacting the branchpoint to promote prespliceosome assembly.

A single polypyrimidine tract binding protein (PTB) binding site mediates splicing inhibition at mouse IgM exons M1 and M2.

Splicing of mouse immunoglobulin (IgM) exons M1 and M2 is directed by two juxtaposed regulatory elements, an enhancer and an inhibitor, located within the M2 exon. A primary function of the enhancer is to counteract the inhibitor, allowing splicing to occur. Here we show that the inhibitor contains two binding sites for polypyrimidine tract binding protein (PTB). Mutational analysis indicates that only one of these sites is necessary and sufficient to direct splicing inhibition both in vitro and in vivo. We demonstrate that the difference in activity of the two sites is explained by proximity to the intron. We further show that the presence of the enhancer results in the disruption of the PTB-inhibitor interaction, enabling splicing to occur. In the absence of the enhancer, splicing can be artificially activated by immuno-inhibition of PTB. Collectively, our results indicate that a single PTB binding site can function as an inhibitor that regulates alternative splicing both in vitro and in vivo.