Photoimmunotherapy retains its anti-tumor efficacy with increasing stromal content in heterotypic pancreatic cancer spheroids

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive disease characterized by increased levels of desmoplasia that contributes to reduced drug delivery and poor treatment outcomes. In PDAC, the stromal content can account for up to 90% of the total tumor volume. The complex interplay between stromal components, including pancreatic cancer associated fibroblasts (PCAFs), and PDAC cells in the tumor microenvironment (TME) have a significant impact on prognoses and thus needs to be recapitulated in vitro when evaluating various treatment strategies. This study is a systematic evaluation of photodynamic therapy (PDT) in 3D heterotypic coculture models of PDAC with varying ratios of patient derived PCAFs that simulate heterogenous PDAC tumors with increasing stromal content. The efficacy of antibody-targeted PDT (photoimmunotherapy; PIT) using cetuximab photoimmunoconjugates (PICs) of benzoporphyrin derivative (BPD) is contrasted with that of liposomal BPD (Visudyne(®)), which is currently in PDT clinical trials for PDAC. We demonstrate that both Visudyne(®)-PDT and PIT were effective in heterotypic PDAC 3D spheroids with a low stromal content. However, as the stromal content increases above 50% in the 3D spheroids, the efficacy of Visudyne(®)-PDT is reduced by up to 10-fold, while PIT retains its efficacy. PIT was found to be 10-fold, 19-fold and 14-fold more phototoxic in spheroids with 50%, 75% and 90% PCAFs, respectively, as compared to Visudyne(®)-PDT. This marked difference in efficacy is attributed to the ability of PICs to penetrate and distribute within spheroids with a higher stromal content, whereas Visudyne(®) is restricted to the spheroid periphery. This study thus demonstrates how the stromal content in PDAC spheroids directly impacts their responsiveness to PDT and proposes PIT to be a highly suited treatment option for desmoplastic tumors with particularly high degrees of stromal content.