This article is part of the 2019 Oncology Special Issue of Natural Medicine Journal. Read the full issue here.
Scatena C, Roncella M, Di Paolo A, et al. Doxycycline, an inhibitor of mitochondrial biogenesis, effectively reduces cancer stem cells (CSCs) in early breast cancer patients: a clinical pilot study. Front Oncol. 2018;8:452.
This clinical pilot study tested whether short-term preoperative treatment with doxycycline reduced cancer stem cell (CSC) activity in breast cancer patients.
A total of 15 women with early breast cancer participated. Nine patients received doxycycline for a 14-day period between breast biopsy and lumpectomy. Six post-lumpectomy specimens were used as controls (no treatment). Controls were chosen from women who were well-matched for age and clinical characteristics.
In the doxycycline treatment group, patient ages at diagnosis ranged from 42 to 65, tumor size varied between 10 and 30 mm, and 7 out of 9 patients were estrogen receptor (ER)–positive (ER+), with 6 being of the luminal A subtype and one of the luminal B subtype. Six out of 9 patients were grade 2, or intermediate in Ki-67. In addition, 2 patients were of the HER2(+) subtype.
The women were given 200 mg per day of doxycycline orally for 14 days before surgery, with breast biopsy serving as the baseline.
Testing was performed on the biopsy and post-resection specimen of each participant, and comparisons were made between measures for each sample. All samples were tested for known biomarkers of “stemness” (CD44, ALDH1); mitochondria (TOMM20); cell proliferation (Ki-67, p27); apoptosis (cleaved caspase-3); and neo-angiogenesis (CD31). Changes from baseline to posttreatment were assessed with MedCalc 12 (unpaired t-test) and ANOVA.
Post-doxycycline tumor samples demonstrated a statistically significant decrease in the stemness marker CD44 (P<0.005) when compared to pre-doxycycline tumor samples. Levels of CD44 were reduced between 17.65% and 66.67% in 8 out of 9 patients treated with doxycycline. One patient showed a 15% increase in CD44. Overall, this represents a nearly 90% positive response rate. Similar results were obtained for ALDH1, another marker of stemness.
There were no changes in any of the biomarkers measured in the control group specimens from time of biopsy to resection, thus the biopsy itself is not likely to have affected measures of stemness.
Cancer stem cells are well-recognized to confer treatment resistance and perhaps give rise to the tumor itself. As Dawood and colleagues summarized in a review of the subject in 2014:
“Cancer stem cells have been identified in a number of solid tumors, including breast cancer, brain tumors, lung cancer, colon cancer, and melanoma. Cancer stem cells have the capacity to self-renew, to give rise to progeny that are different from them, and to utilize common signaling pathways. Cancer stem cells may be the source of all the tumor cells present in a malignant tumor, the reason for the resistance to the chemotherapeutic agent used to treat the malignant tumor, and the source of cells that give rise to distant metastases.”1
To understand the implications of these results, we should review some of the earlier work these researchers published prior to this study.
In 2015, Michael Lisanti reported that antibiotics, which target mitochondria, can eradicate cancer stem cells in multiple types of cancer. In other words, it is possible “to treat cancer like an infectious disease.”2 They had first assessed cancer stem cells from multiple tumor types and “identified a conserved phenotypic weak point—a strict dependence on mitochondrial biogenesis for the clonal expansion and survival of cancer stem cells.”
Simply adding vitamin C and berberine during their doxycycline course of treatment might enhance any anticancer effects.
Their analysis revealed that mitochondria of stem cells might be the Achilles heel of stem cells. Aware that several classes of antibiotics inhibit mitochondrial biogenesis, they next identified a list of drugs that could eradicate cancer stem cells in 12 different cancer cell lines and across 8 different tumor types (ie, breast, ductal carcinoma in situ, ovarian, prostate, lung, pancreatic, melanoma, glioblastoma).2 That same year these researchers identified doxycycline as the preferred drug to use in targeting cancer stem cell mitochondria.3
The US Food and Drug Administration first approved doxycycline as a broad-spectrum antibiotic in 1967. The standard dose is 200 mg/day. Recall that in an evolutionary sense, mitochondria are descendants of bacteria and remain sensitive to the antibiotics more often employed to inhibit bacterial growth.4
Doxycycline is already used to treat infections in cancer patients and there have been case reports of unexpected remissions, particularly in lymphoma.5,6
In April 2017 Zhang et al delineated doxycycline’s action inhibiting the transitional steps of stem cell phenotypes into breast cancer.7
In June 2017 this research took a turn that many of us will find fascinating: Lisanti’s group reported that the effect of doxycycline is optimized when combined with vitamin C and berberine in vitro. (The breast cancer patients in the trial being reviewed here only received doxycycline. Vitamin C and berberine were not included in the study protocol.) Doxycycline is so effective at suppressing cancer stem cell populations that it creates high selection pressure that synchronizes the surviving cancer cell population to a predominantly glycolytic phenotype, which results in metabolic inflexibility. They identified 2 natural products (ie, vitamin C, berberine) and 6 clinically approved drugs (ie, atovaquone, irinotecan, sorafenib, niclosamide, chloroquine, stiripentol) that target the doxycycline-resistant CSC population. This combination strategy eliminates surviving cancer stem cells, which the researchers say provides “a simple pragmatic solution to the possible development of doxycycline-resistance in cancer cells.”8 This earlier in vitro work suggested that doxycycline not only inhibits CSCs but may work best when combined with agents that exploit the metabolic inflexibility, such as vitamin C and berberine.8
It was in light of these earlier publications that the small clinical trial discussed in this review was published. This pilot study suggests that doxycycline at commonly prescribed doses may lessen the “stemness” of tumors in women with breast cancer.
These results suggest, but do not prove, efficacy. The significant decrease in “stemness” observed is not proof that doxycycline will reduce risk of recurrence or slow progression of advanced cancer in the real world. Yet given the safety profile of doxycycline, it is tempting to employ this treatment strategy before definitive evidence is published. Of note, an April 2019 publication suggested that adding azithromycin might further enhance the effectiveness of a doxycycline and vitamin C combination.9
These publications suggest some obvious implications. Patients will, on occasion, take a course of doxycycline to treat infection. This might be a useful opportunity. Simply adding vitamin C and berberine during their doxycycline course of treatment might enhance any anticancer effects. There is no published evidence that doing so will lower risk of cancer or its recurrence, but could it hurt?
Such prophylaxis might be particularly useful in patients previously treated for cancers whose recurrences we suspect are driven by cancer stem cells. Glioblastoma and ovarian cancer come to mind.
In recent years some practitioners have promoted treatment strategies that are the direct opposite of Lisanti’s approach. The thinking is that mitochondrial injury is responsible for cancer progression and so supplements selected to repair mitochondrial injury should be of benefit.10 These 2 approaches are in such direct opposition to one another that one might be justified in thinking that both ideas cannot be true. It is possible that prevention of cancers involves preservation of mitochondria while the existence of established cancer should be considered as a distinctly different state for the cells and their mitochondria.11
In an August 2019 paper, oxidative phosphorylation itself is identified as a potential therapeutic target for cancer therapy.12
Based on published evidence available at this point, once cancer stem cells are present, Lisanti’s argument to target mitochondrial biogenesis is persuasive. It is certainly more well-evidenced than any suggestions that nourishing and promoting mitochondrial biogenesis is useful, even if the latter appears more philosophically congruent with vis medicatrix naturae.