Pau d’arco contains a plant chemical named lapachol which has documented antimalarial, antiseptic, antiviral, bactericidal, fungicidal, insecticidal, pesticidal, schistosomicidal, termiticidal, and virucidal actions. Another chemical in the bark, beta-lapachone, has been demonstrated in laboratory studies to have antibacterial, antifungal, and antiviral actions. Antimicrobial properties of many of Pau d’Arco’s other active phytochemicals were demonstrated in several laboratory studies, in which they exhibited strong in vitro activity against bacteria, fungi, and yeast (including Candida, Aspergillus, Staphylococcus, Streptococcus, Helicobacter pylori, Brucella, tuberculosis, pneumonia, and dysentery).

In addition to its isolated chemicals, a hot water extract of pau d’arco demonstrated antibacterial actions against Staphylococcus aureus, Helicobacter pylori, and Brucella. In other in vitro clinical research an extract of the bark was shown to have strong activity against 11 fungal and yeast strains. Pau d’Arco and its chemicals also have demonstrated in vitro antiviral properties against various viruses, including Herpes I and II, influenza, polio virus, and vesicular stomatitis virus. 

In the 1960s, extracts of Pau d’Arco demonstrated marked antitumorous effects in animals, which drew the interest of the National Cancer Institute (NCI). Researchers decided that the most potent single chemical for this activity was a naphthoquinone chemical named lapachol and they concentrated solely on this single chemical in their subsequent cancer research. In a 1968 study, lapachol demonstrated highly significant activity against cancerous tumors in rats.

By 1970, NCI-backed research already was testing lapachol in human cancer patients. The institute reported, however, that their first Phase I study failed to produce a therapeutic effect without side effects—and they discontinued further cancer research shortly thereafter. These side effects were nausea and vomiting and anti-vitamin K activity. Interestingly, other chemicals in the whole plant extract (which, initially, showed positive antitumor effects at very low toxicity) demonstrated positive effects on vitamin K and, conceivably, compensated for lapachol’s negative effect. Once again, instead of pursuing research on a complex combination of at least 20 active chemicals in a whole plant extract (several of which had anti-tumor effects and other positive biological activities), research focused on a single, patentable chemical—and it didn’t work as well. Despite NCI’s abandonment of the research, another group developed a lapachol analog (which was patentable) in 1975. One study reported that this lapachol analog increased the life span of mice inoculated with leukemic cells by over 80%. In a small, uncontrolled, 1980 study of nine human patients with various cancers (liver, kidney, breast, prostate, and cervix), pure lapachol was reported to shrink tumors and reduce pain caused by them—and three of the patients realized complete remissions.

Another chemical in pau d’arco, beta-lapachone, has been studied closely of late and a number of recent patents have been filed on it. It has demonstrated in laboratory studies to have activities similar to lapachol (antimicrobial, antifungal, antiviral, antitumorous, antileukemic, and anti-inflammatory), with few side effects. Research published from 2003 to 2005 provides important new insights into the possible molecular mechanisms of the anti-cancer activity of beta-lapachone specifically against prostate, colon, pancreatic, and lung cancers. In a 2002 U.S. patent, beta-lapachone was cited to have significant anticancerous activity against human cancer cell lines including: melanoma, promyelocytic leukemia, prostate, malignant glioma, colon, hepatoma, breast, ovarian, pancreatic, multiple myeloma cell lines and drug-resistant cell lines. In yet another U.S. patent, beta-lapachone was cited with the in vivo ability to inhibit the growth of prostate tumors. 

Above are excerpts from The Healing Power of Rainforest Herbs by Leslie Taylor at rain-tree.com. All available third-party research on pau d’arco can be found at PubMed/Medline.