Objective To investigate the role of cellular autophagy in medullary thyroid carcinoma (MTC) under doxorubicin (DOX)-induced stress and chemoresistance and to evaluate the synergistic antitumor effect of combining the autophagy inhibitor chloroquine (CQ) with DOX.

Methods The human MTC cell line TT was used as an in vitro model and assigned to four groups: control, DOX, CQ, and DOX+CQ combination. Autophagy flux was assessed by Western blot analysis of LC3-Ⅱ protein expression, whereas autophagic structures were observed through transmission electron microscopy (TEM). An MTC xenograft model was established in nude mice, and tumor-bearing mice were randomly allocated to corresponding treatment groups. Tumor growth volume and weight were monitored, and autophagy levels in tumor tissues were detected.

Results In vitro experiments showed that DOX treatment considerablyincreased LC3-Ⅱ levels, indicating autophagy induction. The combination of DOX and CQ led to further LC3-Ⅱaccumulation, and TEM revealed numerous undegraded autophagosomes and abnormal autolysosomes, confirming the successful blockade of autophagic flux. In vivo, the DOX+CQ combination therapy inhibited tumor growth significantly more effectively than any single-agent treatment (tumor inhibition rate: 60.0%, P<0.01) without causing considerable systemic toxicity. Marked accumulation of LC3-Ⅱ in tumor tissues from the combination group indicated the effective inhibition of autophagic flux in vivo.

Conclusion Functional autophagy flux is a key mechanism underlying DOX chemoresistance in MTC. CQ can effectively reverse this resistance by blocking autophagic flux, and its combination with DOX exhibits a considerable synergistic antitumor effect. These findings suggest a promising therapeutic strategy for the management of advanced MTC.