Baltimore—Findings from a series of preclinical studies show that digoxin (Lanoxin) can affect tumor biology and interfere with tumor growth and may provide a basis for conducting clinical trials to investigate a potential role of the drug in prostate and other cancers, said Gregg L. Semenza, MD, PhD.

In a recently published paper, Dr. Semenza and colleagues from Johns Hopkins University School of Medicine, Baltimore, reported that digoxin was among a limited group of compounds identified as a potent inhibitor of hypoxia-inducible factor-1 (HIF-1) dependent gene transcription in a cell-based screening assay (Proc Natl Acad Sci USA 2008; 105:19579-86). In subsequent studies, digoxin was shown to inhibit synthesis of HIF-1α by cancer cell lines as well as the downstream expression of HIF-1α target genes. Further testing in studies involving both ex vivo cell models and tumor xenograft animal models demonstrated that digoxin had antitumor effects.

Gregg L. Semenza, MD, PhD

In addition, HIF-α overexpression has been associated with a number of human cancers.

"A VHL gene-inactivating mutation that results in constitutive expression of HIF-α has been identified to be the most common cause of clear cell renal carcinoma, and upregulation of HIF-α has been demonstrated in the primary tumor biopsy specimen in a number of other cancers as well, including prostate cancer," Dr. Semenza said.

"There is also epidemiologic evidence associating tumor levels of HIF-α and patient outcomes. Results of a recent study showed that prostate cancer patients who had very high levels of HIF-α in their primary tumor biopsy specimen were more likely to have biochemical evidence of disease progression after prostatectomy or radiotherapy, and in studies of other cancers, including bladder cancer, high levels of HIF-α have been associated with increased mortality."

The initial screening of compounds for inhibitors of HIF-1 was conducted using the Hopkins Drug Library, which represents a collection of 3,120 drugs compiled by Jun Liu, PhD, professor of pharmacology at the university. All drugs contained in this library have entered phase II clinical testing at least, and most are already approved, noted Dr. Semenza.

Handful of compounds show efficacy

"When new drug candidates are evaluated in cell-based pharmacology screening programs, those identified as having the desired activity must then undergo extensive safety testing, and the majority fail to advance into clinical trials. The agents included in this library already have established safety in humans so that clinical trial evaluation of promising candidates can begin relatively quickly," Dr. Semenza said.

The more than 3,100 agents were initially screened for their ability to inhibit HIF-1 by incubating them under low oxygen conditions with a cell line engineered for hypoxia-inducible expression of firefly luciferase. Using a drug concentration of 0.4 µM, only 20 compounds inhibited HIF-1 transcriptional activity by >88%, including digoxin and 10 other cardiac glycosides.

Further investigation focused on digoxin. To test its antitumor activity, the drug was incubated with PC3 prostate cancer cells. In this study, digoxin inhibited expression of HIF-α, mitigated cell growth, and reduced cell survival. In vivo testing was performed in SCID mice implanted with tumor cells. In this model, digoxin inhibited tumor growth whether the animals began treatment prior to tumor cell implantation or after having established tumor xenografts. The tumors from animals treated with digoxin did shown to have lower levels of HIF-1 than were tumors that developed in untreated controls, whereas in animals injected with tumor cells engineered so that digoxin could not interfere with their ability to synthesize HIF-α, digoxin did not prevent tumor growth.

"This latter study was important to establish that HIF-1 is the target of digoxin's anticancer effects," Dr. Semenza said.

It is unclear how the doses of digoxin that demonstrated efficacy in the animal models compare with the doses that are used to treat patients with heart failure, Dr. Semenza observed. Recent epidemiologic evidence has associated clinical use of digoxin with decreased incidences of leukemia and lymphoma, kidney cancer, urinary tract cancers, and a lower breast cancer mortality rate compared with patients who were not receiving these medications.