Systemic complement protein levels as biomarkers of chemoradiotherapy response in anal squamous cell carcinoma

Background: Identification of easily measurable biomarkers that are able to predict locoregional failure or disease progression following chemoradiotherapy (CRT) would enable more personalised cancer management. Anal squamous cell cancer (ASCC) is the most common type of anal cancer, accounting for approximately 90% of all cases. While CRT is the standard of care for most locally advanced anal cancers, treatment failure occurs in up to 30% of patients. However, it is currently difficult to predict which patients will fail to respond to treatment, highlighting the need for predictive biomarkers. This study aims to assess whether plasma levels of complement proteins can serve as potential biomarkers of treatment response. Materials and Methods: Serial peripheral blood samples from ASCC patients (n = 40) were collected before, during, and after CRT, alongside 6-month clinical and radiological outcomes. Using multiplex ELISA-based technology, we assessed levels of 14 complement proteins at baseline, during CRT, and 3 months post-CRT. Additionally, the same technology was used to compare levels of complement analytes in ASCC and in age- and sex-matched patients without a cancer diagnosis. Results: Our data indicate that CRT decreases levels of most complement analytes measured, with intact C2, intact C3, intact C5, C3a, C5a, Ba, Bb, SC5b9, Factor D, Factor H, Factor I, and Factor P decreasing 3 months after treatment specifically in those patients achieving complete responses (all p < 0.05). Moreover, the treatment failure group showed changes indicative of persistent alternative complement pathway activation, with a less pronounced decline following CRT compared to responders. Furthermore, intact C2 and intact C5 levels were significantly higher in ASCC patients compared to age- and sex-matched patients without a cancer diagnosis (both p < 0.005). In contrast, C3a and C4a were expressed at higher levels in patients without cancer diagnosis compared to ASCC patients (both p < 0.02). Importantly, patients in the treatment failure group had elevated baseline (pre-treatment) levels of intact C2 and Factor D compared to those achieving complete response (both p < 0.03). Conclusions: These findings suggest that dysregulation of the complement system, particularly involving the alternative pathway, may be more prevalent in patients with a poor treatment response. Intact C2 and Factor D may represent potential markers of treatment failure.