P1122: EFFICACY OF ALPELISIB IN PI3K-DRIVEN LANGERHANS CELL HISTIOCYTOSIS

Abstract

Background: Langerhans cell histiocytosis (LCH) is an inflammatory myeloid neoplasm characterized by the accumulation of clonal dendritic cells expressing CD1a and CD207 (Langerin). LCH affects both children and adults and typically ranges from an indolent unifocal form to a progressive multisystemic disease. In the past decade, in depth molecular profiling of LCH as well as other histiocytic neoplasms demonstrated that these diseases are fundamentally dependent on somatic MAPK activating mutations, with the canonical V600E BRAF mutation emerging in more than 50% of LCH patients. Nonetheless, MAPK independent driver mutations were previously reported and anecdotal evidence of successful targeted treatment of various histiocytoses harboring non-canonical drivers such as RET, ALK and CSF1R were previously described in the literature. Aims: To report a case of PI3K driven LCH treated with an isoform specific PIK3CA inhibitor. Methods: Case summary: a 46 year-old mother of 3 presented with complaints of progressive headaches, polydipsia and polyuria. Following a pathological water deprivation test and identification of pituitary stalk thickening on MRI, a diagnosis of central diabetes insipidus (CDI) was established and desmopressin treatment was initiated. However, 10 months following her CDI diagnosis, the patient began to complain of sub febrile episodes of fever and worsening dry cough. Imaging studies including PET/CT disclosed extensive bilateral reticulo-nodular infiltration, nodules with cystic transformation and ground glass opacity, suspicious of pulmonary LCH. Histological examination of a wedge biopsy specimen obtained from the lungs confirmed the diagnosis of LCH and consequent molecular studies identified the M1043V PIK3CA mutation. During her medical investigation, the patient’s conditioned worsened, with new disease foci emerging – including cervical lymphadenopathy and a lytic vertebral lesion involving D11. The patient then received palliative radiotherapy directed at the D11 lytic lesion followed by escalating systemic therapy with single agent Alpelisib, via the Managed Access Program at Novartis. Response to treatment was assessed clinically and using PET/CT. At the molecular level, the effect of Alpelisib was assessed by measuring PTEN expression levels and cell cycle regulating non-coding RNA molecules by using quantitative real-time polymerase chain reaction (qRT-PCR). Results: Treatment with Alpelisib resulted in rapid amelioration of night sweats within 6 days, decrease in back pain within 3 weeks and normalization of all abnormally FDG avid disease foci within 3 months of treatment. The complete metabolic response observed persists currently more than a year following treatment initiation. Treatment with Alpelisib is well tolerated except for a mild increase in HbA1C levels to 6.7%. Moreover, PTEN, the main regulator of the PI3K pathway was found to be upregulated following 3 and 9 months of treatment, in parallel to downregulation of small non-coding RNAs molecules that regulates its expression. Image:Summary/Conclusion: This is the first study demonstrating that the alpha catalytic subunit of PI3K is a targetable non-canonical driver of LCH.