Linda Benedict, Zhang, Fang, Enright, Frederick M., Koh, Gar Yee, Liu, Dong, Liu, Zhijun | 2/26/2011 1:27:26 AM
Solubility-enhanced reformulation of paclitaxel, an old drug
Zhijun Liu, Gar Yee Koh, Fang Zhang, Duane Jeansonne, Rhett Stout, Dong Liu and Fred Enright
Paclitaxel, which is isolated from the bark of the Pacific yew, is a chemotherapeutic, intravenous drug produced under the names of Taxol and Abraxane. Taxol has been a frontline chemotherapeutic drug for treating ovarian and breast cancers for about 16 years and continues to be prescribed even after losing patent protection. As a chemotherapeutic drug, Taxol has been used to treat millions of cancer patients despite unsatisfactory response rates and a plethora of side effects.
Taxol’s therapeutic potential has been held back by challenges in formulation. This challenge starts at the central issue of poor solubility. Paclitaxel is both fatsoluble and water-insoluble. To address the solubility issue, Taxol’s developer, Bristle-Myers Squibb, composed the drug with Cremophor EL, a surfactant purified from castor oil and dehydrated alcohol. The combination of a surfactant and alcohol allows the fat-soluble paclitaxel to be diluted with water for intravenous infusion.
Although Cremophor EL has been an effective surfactant and used for formulating many other drugs, such as the immunosuppressant drug cyclosporine and the anti-retroviral drug ritonavir, it causes hypersensitivity in some patients. To get around the use of Cremophor, Abraxis BioScience Inc. formulates paclitaxel with human albumin to produce Abraxane, an intravenous drug approved by the Food and Drug Administration in 2005. Abraxane has eliminated a large portion of hypersensitivity reactions in humans. Although never reported with Abraxane, the FDA cautions about the remote possibility of disease transmission because of the use of human-derived albumin in the product.
The challenge is how to formulate and deliver paclitaxel to the tumor site in a therapeutic quantity, referred as bioavailability, without causing unwanted side effects. The challenge had been largely unmet until a solubilizing property was discovered in the LSU AgCenter’s Medicinal Plant Laboratory research program. During the process of finding bioactive molecules from plants, one of nature’s well-guarded secrets in solubilizing water-insoluble compounds was also discovered. One single small molecule called rubusoside, part of a compound responsible for the sweet taste of the leaves of the stevia or Rubus plant, was identified and shown to have remarkable and generally applicable solubilizing properties.
In preliminary experiments with a variety of water-insoluble compounds, rubusoside alone increased solubility from two-fold to about one million-fold for vitamin E and propofol. When it was applied to paclitaxel, rubusoside enhanced solubility by 5,000 times. Paclitaxel was solubilized from basically zero to 1.8 grams per liter of water by the use of rubusoside alone, completely eliminating the need for alcohol, Cremophor EL or albumin (Figure 1). Although the underlying mechanisms of this remarkable solubilization are unknown, preliminary studies suggest the formation of rubusoside- paclitaxel (PTX) nanostructures in water (Figure 2).
The new formulation is referred as NANO-PTX. A NANO-PTX water solution can be freeze-dried to powder and completely reconstituted in water or saline. This compatibility is highly desirable because it will not only increase shelflife but also provide a variety of dosage options for both liquid and solid forms.
The solubility enhancement achieved for paclitaxel is encouraging, and its medical relevance appears promising. In proof-of-concept experiments, the cytotoxicity (an indicator of killing power) of NANO-PTX was completely maintained. First, NANO-PTX showed it can inhibit the growth of human pancreatic, breast and colon cancer cells. Second, when orally administered to normal mice, NANO-PTX was detected in their plasma, which indicates some levels of oral bioavailability and possible maintenance of structural integrity of paclitaxel in the biological systems. Third, dying cancer cells were observed in tumor-bearing mice orally administered NANO-PTX. This shows some promise of therapeutic bioavailability (Figure 3) via the oral route. Fourth, a good safety profile was demonstrated for rubusoside.
As a sweetening agent rubusoside has been consumed for hundreds of years in beverage teas. In fact, based on a twoyear study in rats, consuming rubusoside at a daily rate of 0.12 ounce per pound of body weight was found to be at the no-observed-effect level. Consuming it as a sweetener does not require a large amount, but to deliver paclitaxel for treating cancer would require a dose much higher than just to sweeten food and beverages. To gain some insight, a preliminary toxicity study in normal mice found that the animals could tolerate up to 0.65 ounces per pound of body weight, if orally administered, or up to 0.33 ounce per pound of body weight, when intravenously administered, without obvious signs of clinical toxicity. These amounts of rubusoside can deliver more-than-therapeutic amounts of paclitaxel orally or intravenously.
Still, there are more questions than answers. Although research efforts have increased knowledge about rubusoside, developing new knowledge will require a wide spectrum of expertise and financial resources. Because of this demand, the LSU Ag- Center’s Medicinal Plant Laboratory has taken a new research direction. This includes multiple-disciplinary collaborations with two highly accomplished chemistry professors at LSU to characterize the underlying mechanisms of solubilization and cellular uptake. In another collaborative project, LSU AgCenter scientists have assembled expertise in polymer structure characterization, cell transport and anti-tumor efficacy at the LSU Health Sciences Center and the M.D. Anderson Cancer Center in Houston. A third proposed project will investigate the NANO-PTX in canine cancer patients brought to the LSU School of Veterinary Medicine’s companion animal clinic.
The LSU AgCenter has provided small but important seed funds to develop some of the proof-of-concept data. Major funding is needed to move this novel discovery and technology to the forefront of pharmaceutical formulations. The prospect of reformulating an old drug is bright, as the NANO-PTX formulation shows, and reformulation could be applied to other chemotherapeutic agents facing similar problems. Already a new intravenous formulation that uses rubusoside to replace alcohol, Cremophor EL or albumin shows some promises. And even more encouraging is the hoped-for oral paclitaxel medication, which has never been available because of formulation limitations. Making this a success will affect human lives and provide economic development opportunities.
The LSU AgCenter has filed patent protection for the discovery of rubusoside and other natural solubilizers and made initial contacts with several major pharmaceutical companies about the new solubilization technology in reformulating old drugs. In addition to the use in pharmaceutical industry, foods, beverages, agricultural chemicals and functional foods could also apply this technology.
Zhijun Liu, Professor; Gar Yee Koh, Research Associate; Fang Zhang, Duane Jeansonne and Dong Liu, Postdoctoral Researchers, Medicinal Plant Laboratory, School of Renewable Natural Resources; Rhett Stout, Associate Director and Chief Clinical Veterinarian, Division of Laboratory Animal Medicine and Assistant Clinical Professor, LSU School of Veterinary Medicine; and Fred Enright, Professor Emeritus, Department of Veterinary Science.
(This article was published in the winter 2011 issue of Louisiana Agriculture Magazine.)