Baseline Absolute Monocyte Count Predicts Lung Function Decline Among Patients with Systemic Sclerosis-associated Interstitial Lung Disease: A Post Hoc Analysis from the FocuSSced Trial

Objective: Interstitial lung disease (ILD) is the leading cause of death in adults with systemic sclerosis (SSc). The identification of biomarkers to predict progression of SSc-ILD is an important unmet need. The purpose of this study was to determine whether an elevated baseline absolute monocyte count (AMC) is associated with a decline in forced vital capacity (FVC) at 48 weeks among participants with SSc-ILD enrolled in the phase 3 focuSSced trial.

Methods: We performed a post-hoc analysis of the focuSSced trial, a multicenter, double-blind, randomized, placebo-controlled trial of adults with diffuse cutaneous SSc for ≤ 60 months. Participants received subcutaneous tocilizumab 162 mg or placebo weekly for 48 weeks. We examined the relationship between baseline AMC and FVC at 48 weeks using a General Linear Model adjusted for potential confounders.

Results: The 136 participants with SSc-ILD in focuSSced were included in this study. Among participants assigned to the placebo group, there was a statistically significant inverse association between baseline AMC and change in FVC from baseline at week 48 in both unadjusted (β coefficient -0.539, 95 % CI -1.032 to -0.047, p-value=0.032) and multivariable-adjusted (β coefficient -0.573, 95 % CI -1.086 to -0.060, p-value=0.029) models. Among participants with SSc-ILD assigned to the tocilizumab group, there was no statistically significant association between baseline AMC and change in FVC from baseline at week 48 in unadjusted or fully adjusted models.

Conclusion: AMC may be a biomarker of disease progression in SSc-ILD, especially in those with early SSc with elevated circulating inflammatory markers. These results should be validated in other SSc-ILD cohorts.