The dark side of curcumin

Dear Editor, Curcumin is a yellow–orange pigment obtained from the plant Curcuma longa. The powdered rhizome of this plant, called turmeric, is a common ingredient in curry powders and has a long history of use in traditional Asian medicine for a wide variety of disorders. In the last decade a large number of reports have been published on the beneficial effects of curcumin, and it has repeatedly been claimed that this natural product is efficient and safe for the prevention and treatment of several diseases including cancer. It is not surprising, therefore, that curcumin is currently sold as a dietary supplement and that numerous clinical trials are ongoing or recruiting participants to evaluate curcumin activity. But there is accumulating evidence that curcumin may not be so effective and safe. Because such evidence is not generally acknowledged, the purpose of this letter is to briefly review the negative properties of curcumin so that they can be balanced against its beneficial effects. Most of the evidence that supports the therapeutic potential of curcumin is mainly based on in vitro studies in which curcumin was tested at concentrations in the micromolar range. Several reports have demonstrated, however, that the plasma concentrations of curcumin in people taking relatively high oral doses of this compound are very low, typically in the nanomolar range (reviewed in Ref. 4). For instance, a recent study examined the pharmacokinetics of a curcumin preparation in 12 healthy human volunteers 0.25–72 hr after an oral dose of 10 or 12 g. Using a high-performance liquid chromatography assay with a limit of detection of 50 ng mL , only 1 subject had detectable free curcumin at any of the time points assayed. The fact that curcumin also undergoes extensive metabolism in intestine and liver means that high concentrations of curcumin cannot be achieved and maintained in plasma and tissues after oral ingestion. This is a major obstacle for the clinical development of this agent and suggests that the therapeutic potential of oral curcumin is limited. The low clinical efficiency of curcumin in the treatment of several chronic diseases such as Alzheimer’s disease and cardiovascular diseases has been discussed recently. As far as cancer is concerned, in vitro studies have demonstrated that cancer cells do not die unless they are exposed to curcumin concentrations of 5–50 lM for several hours. Because of its poor bioavailability, these concentrations are not achieved outside the gastrointestinal tract when curcumin is taken orally. Because of its extensive metabolism in intestine and liver, these concentrations cannot be maintained for several hours in the gastrointestinal tract. This suggests that the chemotherapeutic potential of oral curcumin is limited even for the treatment of cancers of the gastrointestinal tract. Accordingly, when 15 patients with advanced colorectal cancer were treated with curcumin at daily doses of 3.6 g for up to 4 months, no partial responses to treatment or decreases in tumor markers were observed. A search of the website www.clinicaltrials.gov in July 2009 showed 34 clinical trials using curcumin in a wide variety of diseases, particularly in cancer. In some of these trials, patients with several types of cancer are receiving or will receive curcumin through the oral route. For instance, in an ongoing Phase II clinical trial (NCT00094445), participants with pancreatic cancer are receiving 8 g of curcumin by mouth every day for several 8-week-periods. As discussed before, the plasma concentrations of curcumin in people taking relatively high oral doses of curcumin are very low, typically in the nanomolar range. This means that the oral administration of curcumin does not lead to cytotoxic concentrations outside the gastrointestinal tract. If one assumes that tumor cell death is necessary to achieve an efficient therapeutic response, one should not expect a very positive outcome from this trial. A Phase II Trial is also recruiting participants to test if a daily oral dose of 8 g of curcumin can improve the efficacy of the standard chemotherapy gemcitabine in patients with locally advanced or metastatic adenocarcinoma of the pancreas (NCT00192842). The rationale for this trial is based on in vitro and in vivo data that suggest that noncytotoxic concentrations of curcumin may sensitize cancer cells to the effects of anticancer drugs such as gemcitabine. Although a daily dose of 1 g kg 1 of curcumin increased the antitumor effects of gemcitabine in an orthotopic model of pancreatic cancer, this dose of curcumin (e.g. 70 g in a 70-kg person) is almost 10 times higher than that used in the clinical trial testing the combination of curcumin and gemcitabine (8 g). This makes the outcome of this trial uncertain, as curcumin can either increase or reduce the efficiency of chemotherapy depending on the concentration at which it is used. Several strategies have been proposed to overcome the low oral bioavailability of curcumin. One of these strategies has entered clinical trials and consists of using the black pepper alkaloid piperine (bioperine) to increase the bioavailability of curcumin. This strategy, however, should be used cautiously, as piperine is a potent inhibitor of drug Le tt er s to th e E di to r