Abstract Body (Do not enter title and authors here): Introduction. Pulmonary vein stenosis (PVS) is a rare disease associated with significant morbidity and mortality, and its underlying pathophysiology remains poorly understood. PVS is characterized histologically by intimal hyperplasia and exuberant proliferation of myofibroblasts. Sirolimus, a mammalian target of rapamycin (mTOR) inhibitor, is increasingly employed in PVS patients to decrease myofibroblast proliferation and sirolimus has been associated with improved survival in PVS. Nevertheless, it is unclear how sirolimus alters the pathophysiology of PVS and whether biomarkers of response can be identified.

Hypothesis. Systemic sirolimus treatment alters the circulating proteomic profile in pediatric PVS, which may provide insight into the mechanisms of its therapeutic effect.

Methods. Pediatric PVS patients were prospectively identified and consented for a blood draw at the time of a clinically indicated cardiac catheterization. Clinical data were abstracted from the medical record. Serum was isolated and global, aptamer-based proteomics was performed (SomaScan 11K platform). Samples were grouped by sirolimus treatment and normalized data were analyzed by t-test with adjusted p-value (Wilcoxon) for multiple comparisons. The top 3 differentially regulated proteins were identified, and receiver operating characteristic (ROC) curve and random forest analysis were performed.

Results. Twenty-six samples were collected from 22 pediatric PVS patients (45% male) with a median age of 4.0 years (IQR 2.0 – 6.3). Half of the samples (n=13) were obtained from PVS patients on sirolimus at the time of blood collection with a median trough level of 7.5 ng/ml. BEX4 (log₂fold change -0.48, adjusted p-value 0.047), PCDH9 (log₂fold change -0.97, adjusted p-value 0.047), and CEMIP (log₂fold change -1.12, adjusted p-value 0.047) were significantly downregulated in PVS patients on sirolimus (Figure 1A). In combination, BEX4, PCDH9 and CEMIP result in a ROC curve with an area under the curve of 0.887 (Figure 1B) and strong separation by random forest (Figure 1C).

Conclusions. Serum proteomic analysis in pediatric PVS patients reveals distinct protein expressions between those treated with systemic sirolimus and untreated individuals. Further investigation is warranted to evaluate the translational potential for utilizing circulating protein expression for risk stratification or as a biomarker of therapeutic response to sirolimus in pediatric PVS.