PREVENTING IN VIVO RESISTANCE TO PI3K/AKT/MTOR INHIBITORS THROUGH MEDIATOR KINASE INHIBITION - DYSREGULATION OF THE PI3K/AKT/MTOR PATHWAY IS ONE OF THE MOST COMMON GENETIC ALTERATIONS IN CANCER, FOUND IN MORE THAN ONE THIRD OF ALL SOLID TUMORS. INHIBITORS OF THE KEY COMPONENTS OF THIS PATHWAY (MTOR, PI3K, AKT) HAVE BEEN DEVELOPED; SOME OF THEM HAVE BEEN APPROVED FOR BREAST CANCER AND FOR CERTAIN OTHER MALIGNANCIES. HOWEVER, THE BENEFITS OF THESE INHIBITORS ARE LIMITED BY THE DEVELOPMENT OF RESISTANCE, WHICH ARISES EITHER THROUGH ENZYMATIC CHANGES WITHIN THE PATHWAY OR BY THE ACTIVATION OF COMPENSATORY SIGNAL TRANSDUCTION PATHWAYS OR OTHER TRANSCRIPTIONAL CHANGES. SUPPRESSING TRANSCRIPTIONAL RESISTANCE MECHANISMS COULD REVITALIZE THE USE OF PI3K/AKT/MTOR INHIBITORS, WITH A POTENTIALLY TRANSFORMATIVE IMPACT FOR MANY CANCER PATIENTS. WE PROPOSE TO COMBINE PI3K/AKT/MTOR INHIBITORS WITH A DRUG CANDIDATE THAT SELECTIVELY INHIBITS CDK8/19 MEDIATOR KINASE, A REGULATOR OF TRANSCRIPTIONAL REPROGRAMMING, THE KEY PROCESS REQUIRED FOR THE EMERGENCE OF DRUG RESISTANCE. CDK8/19 INHIBITORS (CDK8/19I) SUPPRESS THE EMERGENCE OF RESISTANCE TO DIFFERENT ANTICANCER DRUGS. IN PRELIMINARY STUDIES, XENOGRAFT TUMORS FORMED BY PTEN-DEFICIENT BREAST CANCER CELLS WERE TREATED WITH MTOR INHIBITOR EVEROLIMUS. ALTHOUGH TUMOR GROWTH WAS FULLY ARRESTED FOR THE FIRST 7-8 WEEKS, ALL THE EVEROLIMUS-TREATED TUMORS SUBSEQUENTLY RESUMED THEIR GROWTH, INDICATING THE DEVELOPMENT OF RESISTANCE. WHILE TREATMENT WITH A CDK8/19I AS A SINGLE AGENT HAD A MODEST EFFECT ON TUMOR GROWTH, WHEN COMBINED WITH EVEROLIMUS, THE CDK8/19I FULLY PREVENTED THE DEVELOPMENT OF RESISTANCE OVER 150 DAYS OF TREATMENT. PREVENTION OF EVEROLIMUS RESISTANCE BY THE CDK8/19I WAS ASSOCIATED WITH THE COUNTERACTION OF TRANSCRIPTOMIC EFFECTS OF EVEROLIMUS ON BOTH TUMOR AND STROMAL CELLS. WE HYPOTHESIZE THAT OUR SELECTIVE CDK8/19I PRECLINICAL DRUG CANDIDATE WILL SUPPRESS THE DEVELOPMENT OF RESISTANCE NOT ONLY TO MTOR BUT ALSO TO PI3K AND AKT INHIBITORS, EXTENDING THE DURATION OF REMISSION AND POSSIBLY PRODUCING CURES. THIS PHASE I SBIR PROGRAM WILL DETERMINE IF THE CDK8/19I PREVENTS THE DEVELOPMENT OF IN VIVO RESISTANCE TO MTOR INHIBITOR EVEROLIMUS, PI3KA INHIBITOR ALPELISIB, AND AKT INHIBITOR CAPIVASERTIB, IN ESTROGEN RECEPTOR (ER)-POSITIVE AND TRIPLE-NEGATIVE BREAST CANCERS. UNDER AIM 1, WE WILL TEST THE ABILITY OF THE CDK8/19I TO PREVENT THE DEVELOPMENT OF IN VIVO RESISTANCE TO EVEROLIMUS, ALPELISIB AND CAPIVASERTIB IN CELL-LINE BASED ER-POSITIVE AND TRIPLE-NEGATIVE BREAST CANCER XENOGRAFTS. IMMUNOHISTOCHEMISTRY AND TRANSCRIPTOMIC ANALYSIS WILL BE USED TO CHARACTERIZE THE TUMOR- AND STROMA-BASED MECHANISMS OF IN VIVO RESISTANCE TO PI3K/AKT/MTOR INHIBITORS, WHICH ARE PREVENTED BY CDK8/19 INHIBITION. UNDER AIM 2, WE WILL DETERMINE IF THE CDK8/19I WILL PREVENT THE DEVELOPMENT OF IN VIVO RESISTANCE TO THE COMBINATION OF EVEROLIMUS AND CDK8/19I IN DIFFERENT CLINICALLY RELEVANT PATIENT-DERIVED XENOGRAFT (PDX) MODELS OF ER-POSITIVE AND TRIPLE- NEGATIVE BREAST CANCERS. IF THE CDK8/19I PREVENTS THE EMERGENCE OF IN VIVO RESISTANCE IN THESE MODELS, THE FUTURE PHASE II STUDIES WILL BE EXPANDED TO OTHER PI3K/AKT/MTOR INHIBITORS, IN BREAST CANCER AND OTHER MALIGNANCIES, TO IDENTIFY THE BEST DRUG COMBINATIONS AND INDICATIONS FOR CLINICAL DEVELOPMENT.