Hydroxychloroquine inhibits autophagy to potentiate antiestrogen responsiveness in ER⁺ breast cancer

Purpose: Estrogen receptor-α (ER)-targeted therapies including tamoxifen (TAM) or Faslodex (ICI) are used to treat ER⁺ breast cancers. Up to 50% of tumors will acquire resistance to these interventions. Autophagy has been implicated as a major driver of antiestrogen resistance. We have explored the ability of hydroxychloroquine (HCQ), which inhibits autophagy, to affect antiestrogen responsiveness. Experimental Design: TAM-resistant MCF7-RR and ICI-resistant/TAM cross-resistant LCC9 ER⁺ breast cancer cells were injected into mammary fat pads of female athymic mice and treated with TAM and/or ICI in combination with oral low-dose HCQ. Results: We show that HCQ can increase antiestrogen responsiveness in MCF7-RR and LCC9 cells and tumors, likely through the inhibition of autophagy.However, the combination of ICI+HCQ was less effective than HCQ alone in vivo, unlike the TAM+HCQ combination. Antiestrogen treatment stimulated angiogenesis in tumors but did not prevent HCQ effectiveness. The lower efficacy of ICI+HCQ was associated with ICI effects on cell-mediated immunity within the tumor microenvironment. The mouse chemokine KC (CXCL1) and IFNγ were differentially regulated by both TAM and ICI treatments, suggesting a possible effect on macrophage development/activity. Consistent with these observations, TAM+HCQ treatment increased tumor CD68⁺ cells infiltration, whereas ICI and ICI+HCQ reduced peripheral tumor macrophage content. Moreover, macrophage elimination of breast cancer target cells in vitro was reduced following exposure to ICI. Conclusion: HCQ restores antiestrogen sensitivity to resistant tumors. Moreover, the beneficial combination of TAM+HCQ suggests a positive outcome for ongoing neoadjuvant clinical trials using this combination for the treatment of ER ⁺ ductal carcinoma in situ lesions.