Radiation Enhancement of Head and Neck Squamous Cell Carcinoma by the Dual PI3K/mTOR Inhibitor PF-05212384.
OBJECTIVE
Radiation remains a mainstay for the treatment of nonmetastatic head and neck squamous cell carcinoma (HNSCC), a malignancy characterized by a high rate of PI3K/mTOR signaling axis activation. We investigated the ATP-competitive dual PI3K/mTOR inhibitor, PF-05212384, as a radiosensitizer in preclinical HNSCC models.


METHODS
Extent of radiation enhancement of two HNSCC cell lines (UMSCC1-wtP53 and UMSCC46-mtP53) and normal human fibroblast (1522) was assessed by in vitro clonogenic assay with appropriate target inhibition verified by immunoblotting. Radiation-induced DNA damage repair was evaluated by γH2AX Western blots with the mechanism of DNA double-strand break repair abrogation investigated by cell cycle analysis, immunoblotting, and RT-PCR. PF-05212384 efficacy in vivo was assessed by UMSCC1 xenograft tumor regrowth delay, xenograft lysate immunoblotting, and tissue section immunohistochemistry.


RESULTS
PF-05212384 effectively inhibited PI3K and mTOR, resulting in significant radiosensitization of exponentially growing and plateau-phase cells with 24-hour treatment following irradiation, and variable radiation enhancement with 24-hour treatment before irradiation. Tumor cells radiosensitized to a greater extent than normal human fibroblasts. Postirradiation PF-05212384 treatment delays γH2AX foci resolution. PF-05212384 24-hour exposure resulted in an evident G1-S phase block in p53-competent cells. Fractionated radiation plus i.v. PF-05212384 synergistically delayed nude mice bearing UMSCC1 xenograft regrowth, with potential drug efficacy biomarkers identified, including pS6, pAkt, p4EBP1, and Ki67.


CONCLUSIONS
Taken together, our results of significant radiosensitization both in vitro and in vivo validate the PI3K/mTOR axis as a radiation modification target and PF-05212384 as a potential clinical radiation modifier of nonmetastatic HNSCC.
Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland. Medical Research Scholars Program, NIH, Bethesda, Maryland.
Andrew J
AJ
Leiker
Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
William
W
DeGraff
Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
Rajani
R
Choudhuri
Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
Anastasia L
AL
Sowers
Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
Angela
A
Thetford
Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
John A
JA
Cook
Head and Neck Surgery Branch, NIDCD, NIH, Bethesda, Maryland.
Carter
C
Van Waes
Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland. jbm@helix.nih.gov.
James B
JB
Mitchell
D000818:Animals; D002294:Carcinoma, Squamous Cell / Q000378:metabolism* / Q000473:pathology / Q000628:therapy; D002453:Cell Cycle / Q000187:drug effects; D045744:Cell Line, Tumor; D053903:DNA Breaks, Double-Stranded / Q000187:drug effects / Q000528:radiation effects; D004260:DNA Repair / Q000187:drug effects / Q000528:radiation effects; D004195:Disease Models, Animal; D004307:Dose-Response Relationship, Radiation; D055785:Gene Knockdown Techniques; D006258:Head and Neck Neoplasms / Q000378:metabolism* / Q000473:pathology / Q000628:therapy; D006801:Humans; D051379:Mice; D009025:Morpholines / Q000493:pharmacokinetics*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D011836:Radiation Tolerance / Q000187:drug effects; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects; D000077195:Squamous Cell Carcinoma of Head and Neck; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014227:Triazines / Q000493:pharmacokinetics*; D047368:Tumor Burden / Q000187:drug effects / Q000528:radiation effects; D023041:Xenograft Model Antitumor Assays
D009025:Morpholines; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D047428:Protein Kinase Inhibitors; D011838:Radiation-Sensitizing Agents; D058570:TOR Serine-Threonine Kinases; D014227:Triazines; C549060:gedatolisib