Macrolide and Ketolide Antibiotics Inhibit the Cytotoxic Effect of Trastuzumab Emtansine in HER2-Positive Breast Cancer Cells: Implication of a Potential Drug-ADC Interaction in Cancer Chemotherapy

Macrolides are widely used for the long-term treatment of infections and chronic inflammatory diseases. The pharmacokinetic features of macrolides include extensive tissue distribution because of favorable membrane permeability and accumulation within lysosomes. Trastuzumab emtansine (T-DM1), a HER2-targeting antibody-drug conjugate (ADC), is catabolized in the lysosomes, where Lys-SMCC-DM1, a potent cytotoxic agent, is processed by proteinase degradation and subsequently released from the lysosomes to the cytoplasm through the lysosomal membrane transporter SLC46A3, resulting in an antitumor effect. We recently demonstrated that erythromycin and clarithromycin inhibit SLC46A3 and attenuate the cytotoxicity of T-DM1; however, the effect of other macrolides and ketolides has not been determined. In this study, we evaluated the effect of macrolide and ketolide antibiotics on T-DM1 cytotoxicity in a human breast cancer cell line, KPL-4. Macrolides used in the clinic, such as roxithromycin, azithromycin, and josamycin, as well as solithromycin, a ketolide under clinical development, significantly attenuated T-DM1 cytotoxicity in addition to erythromycin and clarithromycin. Of these, azithromycin was the most potent inhibitor of T-DM1 efficacy. These antibiotics significantly inhibited the transport function of SLC46A3 in a concentration-dependent manner. Moreover, these compounds extensively accumulated in the lysosomes at the levels estimated to be 0.41-13.6 mM when cells were incubated with them at a 2 μM concentration. The immunofluorescence staining of trastuzumab revealed that azithromycin and solithromycin inhibit the degradation of T-DM1 in the lysosomes. These results suggest that the attenuation of T-DM1 cytotoxicity by macrolide and ketolide antibiotics involves their lysosomal accumulation and results in their greater lysosomal concentrations to inhibit the SLC46A3 function and T-DM1 degradation. This suggests a potential drug-ADC interaction during cancer chemotherapy.

Identification of 5-Carboxyfluorescein as a Probe Substrate of SLC46A3 and Its Application in a Fluorescence-Based In Vitro Assay Evaluating the Interaction with SLC46A3

The therapeutic modalities that involve the endocytosis pathway, including antibody-drug conjugates (ADCs), have recently been developed. Since the drug escape from endosomes/lysosomes is a determinant of their efficacy, it is important to optimize the escape, and the cellular evaluation system is needed. SLC46A3, a lysosomal membrane protein, has been implicated in the pharmacological efficacy of trastuzumab emtansine (T-DM1), a noncleavable ADC used for the treatment of breast cancer, and the cellular uptake efficacy of lipid-based nanoparticles. Recently, we identified the SLC46A3 function as a proton-coupled steroid conjugate and bile acid transporter, which can directly transport active catabolites of T-DM1. Thus, the rapid and convenient assay systems for evaluating the SLC46A3 function may help to facilitate ADC development and to clarify the physiological roles in endocytosis. Here, we show that SLC46A3 dC, which localizes to the plasma membrane owing to lacking a lysosomal-sorting motif, has a great ability to transport 5-carboxyfluorescein (5-CF), a fluorescent probe, in a pH-dependent manner. 5-CF uptake mediated by SLC46A3 was significantly inhibited by compounds reported to be SLC46A3 substrates/inhibitors and competitively inhibited by estrone 3-sulfate, a typical SLC46A3 substrate. The inhibition assays followed by uptake studies revealed that SG3199, a pyrrolobenzodiazepine dimer, which has been used as an ADC payload, is a substrate of SLC46A3. Accordingly, the fluorescence-based assay system for the SLC46A3 function using 5-CF can provide a valuable tool to evaluate the interaction of drugs/drug candidates with SLC46A3.

SLC46A3 is a lysosomal proton-coupled steroid conjugate and bile acid transporter involved in transport of active catabolites of T-DM1

Antibody-drug conjugates (ADCs) represent a new class of cancer therapeutics that enable targeted delivery of cytotoxic drugs to cancer cells. Although clinical efficacy has been demonstrated for ADC therapies, resistance to these conjugates may occur. Recently, SLC46A3, a lysosomal membrane protein, was revealed to regulate the efficacy of trastuzumab emtansine (T-DM1), a noncleavable ADC that has been widely used for treating breast cancer. However, the role of SLC46A3 in mediating T-DM1 cytotoxicity remains unclear. In this study, we discovered the function of SLC46A3 as a novel proton-coupled steroid conjugate and bile acid transporter. SLC46A3 preferentially recognized lipophilic steroid conjugates and bile acids as endogenous substrates. In addition, we found that SLC46A3 directly transports Lys-SMCC-DM1, a major catabolite of T-DM1, and potent SLC46A3 inhibitors attenuate the cytotoxic effects of T-DM1, suggesting a role in the escape of Lys-SMCC-DM1 from the lysosome into the cytoplasm. Our findings reveal the molecular mechanism by which T-DM1 kills cancer cells and may contribute to the rational development of ADCs that target SLC46A3.

Increased expression of SLC46A3 to oppose the progression of hepatocellular carcinoma and its effect on sorafenib therapy

Hepatocellular carcinoma (HCC) prognosis remains dismal due to postsurgical recurrence and distant metastasis. Therefore, novel prognostic biomarkers and therapeutic targets for HCC therapy are urgently needed to improve the survival of liver cancer patients. Our evidence suggests that SLC46A3 (the gene solute carrier family 46 (sodium phosphate), member 3) is a member of the SLC46 family and has a potential role in the progression and treatment of HCC. The objective of the present study was to estimate the expression pattern and biological function of SLC46A3 in the progression of HCC, which may serve as a promising biomarker for diagnosis and therapy. In order to determine the expression pattern of SLC46A3 in HCC, several public HCC databases and tissue chips were used to examine 129 sets of primary HCC and non-tumor adjacent tissues from patients who had undergone surgery. The expression of SLC46A3 in 80 sets of HCC and non-tumor adjacent tissues were then compared by RT-PCR and Western Blot. The proliferation, invasion, migration and sphere-forming abilities of SLC46A3 knock-down and overexpressing cell lines were evaluated and the expression of related molecules in the epithelial mesenchymal transition (EMT) were detected by RT-PCR, western blot and immunofluorescence assay. The IC₅₀ value was used to evaluate the effect of SLC46A3 on sorafenib resistance. A lung metastasis model of mice HCC was constructed to test the potential effect of SLC46A3 on cancer metastasis and a subcutaneous xenografted tumor mice model was designed to verify the effect of SLC46A3 on the resistance of HCC cell lines to sorafenib. The expression of SLC46A3 was down-regulated in 83.2% of human HCC tissues compared to non-tumor adjacent tissues. Tumors that expressed low levels of SLC46A3 had more aggressive phenotypes, and patients with these tumors had shorter survival times after surgery compared to patients whose tumors expressed high levels of SLC46A3. Hepatocellular carcinoma cell lines that stably overexpressed SLC46A3 inhibited the levels of migration and invasion compared with control HCC cells, and formed smaller xenograft tumors with more metastases in mice compared with HCC cells that did not overexpress SLC46A3. In addition, overexpression of SLC46A3 obviously inhibited epithelial-to-mesenchymal transition-activating transcription factors such as N-cadherin and Vimentin. Furthermore, descended of IC₅₀ showed that overexpressed SLC46A3 could reduce sorafenib resistance and improve drug response in vivo and in vitro. In conclusion, increased expression of SLC46A3 could favor a better clinical prognosis for patients with HCC, ameliorate sorafenib resistance, and improve drug response. SLC46A3 might serve as a potential prognostic biomarker and therapeutic target in HCC.