Research on Doxycycline in the Treatment of Malignant Solid Tumors

Research on DOX in malignant solid tumors

DOX has been widely studied in a variety of malignant solid tumors and has shown good tumor suppressor effects. Here, this article briefly reviews the application of DOX in common clinical solid tumors including breast cancer (BRCA), colorectal cancer (CRC), prostate cancer (PRAD), and melanoma.

• Breast Cancer

In BRCA-related research, DOX mainly inhibits tumor proliferation, migration and invasion by targeting breast cancer stem cells (BCSCs). Studies have shown that DOX can inhibit the proliferation activity of breast cancer cells and BCSCs, and down-regulate the expression of stem cell factors Oct4, Sox2, Nanog and CD44. Thereby reducing the spheroidization efficiency, migration and invasion of BCSCs and the epithelial-mesenchymal transition (EMT) of breast cancer cells. The study found that DOX can act as a mitochondrial inhibitor, effectively targeting BCSCs and significantly reducing the frequency and spheroidization efficiency of aldehyde dehydrogenase-positive (ALDH+) BCSCs in HER2+ and triple-negative breast cancer (TNBC) subtypes. In addition, as an inhibitor of MMPs, DOX is closely related to inhibiting tumor metastasis. In an experimental bone metastasis mouse model of human breast cancer MDA-MB-231 cells, DOX treatment resulted in a 70% reduction in total tumor burden and enhanced bone resorption, indicating that DOX treatment may improve the risk of osteolytic bone metastasis in patients with osteolytic bone metastases. Breast cancer patients at risk of osteolytic bone metastases may benefit. However, some studies believe that DOX combination therapy cannot relieve bone pain symptoms in patients with breast cancer bone metastasis, and bone metastasis markers have not been significantly changed. In studies on the mechanism, studies have confirmed that DOX inhibits the progression of breast cancer by directly targeting protease-activated receptor 1 (PAR-1), and found that miR-17 can reverse the partial inhibition effects of DOX on breast cancer both in vivo and in vitro.

• Colorectal Cancer

Colorectal cancer (CRC) is the second most common solid malignant tumor in humans. Among them, tumor metastasis is the main cause of death and is considered to be the main challenge in CRC treatment. In this context, studies on human HT29 colorectal cancer cell lines have shown that DOX can inhibit tumor growth and progression through cytotoxic effects on tumor cells, inhibition of MMPs, and induction of apoptosis. At the same time, DOX can also reduce the apoptosis threshold of HT29 cells by targeting mitochondria, thereby promoting the apoptosis of colorectal cancer cells. In addition, studies have found that in CRC, DOX can inhibit tumor progression in combination with other anti-tumor drugs, anti-angiogenic factors and anti-checkpoint blockers. In terms of gene therapy, DOX can regulate the expression of human interleukin 12 (IL-12) gene and achieve high expression of precise target cytokines in CRC. This illustrates the important potential of the DOX-adenoviral vector system in gene therapy of CRC and other malignant tumors.

Figure 1. Function of doxycycline in colorectal cancer.(Ghasemi K, et al.; 2022)

• Prostate Cancer

PRAD is the most common type of cancer among older men. The study found that an increase in apoptotic cells was confirmed in the human prostate cancer cell line LNCaP treated with DOX, indicating that DOX may have a potential role in the treatment of prostate cancer. DOX combined with doxorubicin can significantly inhibit PC3 cell lines in 2D and 3D culture media, enhance cell apoptosis, and increase cell cycle arrest in the G2/M phase. After combined treatment, RT-PCR and Western blot tests showed down-regulation of Bcl-2 and up-regulation of Bax, indicating that the combination of DOX and doxorubicin has a synergistic effect on PC3 cells and may provide potential new treatments for castration-resistant prostate cancer strategy. Research results show that DOX can inhibit the initiation of NLR family Pyrin domain protein 3 (NLRP3) in PC3 cell lines and alleviate the progression of PRAD by increasing early apoptosis of tumor cells.

• Osteosarcoma

Osteosarcoma is a highly aggressive primary bone tumor that mainly affects young and middle-aged people. Studies have shown that DOX blocks extracellular matrix (ECM) and membrane degradation by inhibiting MMPs. Make osteosarcoma cells lose their ability to invade and migrate. In addition, DOX eliminates the secretion of vascular endothelial growth factor (VEGF) and deprives the supply of nutrients through its anti-angiogenic effect. In human osteosarcoma OSA cell lines, DOX intervention can inhibit cell proliferation and MMPs activity, and induce OSA cell apoptosis. Evidence suggests that this well-tolerated oral agent may be effective in the clinical treatment of osteosarcoma.

oral squamous cell carcinoma

Regional lymph node and distant organ metastasis of oral squamous cell carcinoma (OSCC) are closely related to the increased expression of MMPs, and DOX, as an inhibitor of MMPs, has unique advantages in the treatment of OSCC. In in vitro experiments, DOX was found to inhibit the invasion and migration of SCC-15 cell lines by downregulating the expression of MMP-2 and MMP-9. However, in contrast, studies have also shown that DOX-induced expression of exogenous B cell-specific Moloney murine leukemia virus integration site 1 (Bmi-1) enhances tumorigenesis in the OSCC transgenic mouse line KrTBmi-1 model form.

• Lung Cancer

Data demonstrate that functional targeting of transcription factors by DOX reverses EMT and inhibits proliferation and migration of the human small cell lung cancer cell line NCI-H446. Therefore, DOX selectively targets malignant tumors and reduces their metastatic potential while having lower cytotoxicity in lung cancer patients. It is worth noting that small cell lung cancer (SCLC) is a highly aggressive tumor type with poor prognosis. DOX dose-dependently inhibits the proliferation, colony formation, migration and invasion of human SCLCH446 cell line, as well as by upregulating caspase- 3 and bax expression, as well as down-regulating the expression of survivin and bcl-2 to induce apoptosis. These findings support further investigation of DOX's potential as a drug candidate for the treatment of SCLC.

• Melanoma

A mouse subcutaneous tumor-bearing model was established by subcutaneous injection of B16 melanoma cell suspension. Study found that the combined treatment of melanoma with DOX and endostatin can affect the formation of three microcirculation patterns: endothelium-dependent blood vessels, mosaic blood vessels and angiogenic mimicry in melanoma, reducing tumor volume and slowing down growth. Notably, DOX also inhibits the expression and phosphorylation of focal adhesion kinase (FAK), a protein tyrosine kinase involved in regulating cell adhesion and migration. DOX is most likely to exert its anti-tumor effect by inhibiting the FAK signaling pathway. These results provide promising prospects for the potential application of DOX in anti-tumor therapy and deserve further study.

• Ovarian Cancer

Ovarian cancer often has a poor prognosis due to its typical aggressive metastasis and treatment resistance. Studies have shown that DOX not only inhibits the epithelial ovarian cancer cell line SKOV3, but also increases the sensitivity of SKOV3 to cisplatin. The activation of AKT and ERK1/2 signaling pathways regulated by SDF-1α/CXCR4 may be involved in the anti-tumor effect of DOX on SKOV3 cells.

• Pancreatic Cancer

In in vitro experiments on the human pancreatic cancer cell line PANC-1, DOX continuously activated the transcription of p53, p21 and cascade-related genes, while reducing the expression of Bcl-2, Mcl-1 and paclitaxel-induced IL-8. In a mouse xenograft model, DOX treatment inhibited tumor growth by 80%. It shows that DOX exerts anti-pancreatic cancer effects by activating pro-apoptotic genes, inhibiting IL-8 expression and inhibiting anti-apoptotic genes. Pancreatic cancer patient data obtained from TCGA indicate that high PAR1 and FAK protein expression predicts poor patient prognosis. PAR1 can regulate FAK, phosphatidylinositol 3-kinase (PI3K) phosphorylation and EMT in human pancreatic cancer cell lines ASPC-1 and PANC-1. As a PAR1 inhibitor, DOX can effectively inhibit the CSCs-like properties of pancreatic cancer cells and the activation of the FAK/PI3K/AKT pathway.

• Other Malignant Solid Tumors

DOX also reduces biological processes such as proliferation, migration, invasion and EMT of cancer cells from different sources. In cervical cancer, DOX inhibits the proliferation, invasion and differentiation of human cervical cancer cell line HeLa-CSCs in vitro and in vivo, reduces stem cell markers SOX-2, OCT-4, NANOG, NOTCH and BMI-1, and induces cell apoptosis.