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Antibiotic exposure is associated with worse clinical outcomes in patients receiving immune checkpoint inhibitors (ICI). We analyzed antibiotic prescription patterns in lung cancer and melanoma, two malignancies in which ICI are used broadly across stages.We performed a retrospective cohort study of adults in the U.S. Veterans Affairs (VA) medical system diagnosed with lung cancer or melanoma from 2003 to 2016. We defined antibiotic exposure as receipt of a prescription for a systemic antibacterial agent between 6 months before and 6 months after cancer diagnosis. Demographics, clinical variables, prescriptions, and diagnostic codes were abstracted from the VA Corporate Data Warehouse. Antibiotic exposure was compared using t tests, Chi-square, and multivariate analyses.A total of 310,321 patients (280,068 lung cancer, 30,253 melanoma) were included in the analysis. Antibiotic exposure was more common among patients with lung cancer (42% vs. 24% for melanoma; P.001). Among antibiotic-exposed patients, those with lung cancer were more likely to receive prescriptions for multiple antibiotics (47% vs. 30% for melanoma; P.001). In multivariate analyses, antibiotic exposure was associated with lung cancer diagnosis (HR 1.50; 95% CI, 1.46-1.55), comorbidity score (HR 1.08; 95% CI, 1.08-1.09), non-white race (HR 1.11; 95% CI, 1.06-1.17), and female gender (HR 1.31; 95% CI, 1.24-1.37).Among cancer patients, antibiotics are prescribed frequently. Antibiotic exposure is more common in certain cancer types and patient populations. Given the negative effect antibiotic exposure has on immunotherapy outcomes, these observations may have clinical and healthy policy implications.

Currently every new non-small cell lung cancer (NSCLC) patient being evaluated for specific treatment has a history, physical exam, a series of radiologic procedures (computed tomographic and often PET scans), standard blood work, and evaluation of their lung cancer tumour biopsy to establish a histologic diagnosis of lung cancer, to help determine the individual patient's prognosis, and to select an appropriate treatment regimen. Key clinical elements going into these decisions are the type of lung cancer (e.g. non-small cell vs. small cell, adenocarcinoma vs. squamous cancer), the stage of the tumour (e.g. localized or metastatic), patient performance status, and co-morbidities. In the past decade patients also receive an evaluation of tumour molecular characteristics (by DNA sequencing of a tumour biopsy for mutations) that could lead to targeted therapy (e.g. presence of an EGFR mutation leading to EGFR tyrosine kinase inhibitor therapy). In addition, with the demonstration of significant survival benefit from immune checkpoint inhibitor (ICI) therapy in a subset (15–20%) of non-small cell lung cancers (NSCLCs) there is a concerted effort to determine which molecular and immunologic characteristics of the patient and the tumour or its microenvironment predict response to immunotherapy [1]. Finally, there has been a decades long search for tumour molecular characteristics (e.g. mRNA or protein expression) that provide prognostic information over and above that provided by standard clinical data. While such prognostic information is important, it does not provide much solace to tell patients “you have lung cancer which is a bad cancer, and our molecular analyses shows us you have a really bad type of lung cancer.” Thus, for benefit to patients, any laboratory analyses for tumour characteristics should focus on characteristics that would help deliver a better therapy for that individual patient.