Solubilization of the lichen metabolite (+)-usnic acid for testing in tissue culture.
The pharmacological testing of natural products can often be hampered by the poor solubility of such compounds in non-toxic solvents. There is thus a need for a suitable agent for solubilization of natural substances to allow testing on a variety of cell lines in-vitro. Such an agent should ideally have no direct effects on any of the commonly used cell lines from a variety of tissues and mammalian species to allow proper comparison. In this study, the lichen metabolite (+)-usnic acid, a dibenzofuran derivative, was used as a prototype for an insoluble natural product with the aim of finding a solvent that was both capable of solubilizing usnic acid and was free of direct activity against a test cell line. Solubilization was measured at different pH values in various concentrations of co-solvents (glycofurol 75, propylene glycol, polyethylene glycol 400), surfactants (polysorbate 20 and Cremophor RH40), and the complexing agent 2-hydroxypropyl-beta-cyclodextrin. The solubility achieved in a 20% aqueous solution was 0.11 mg mL(-1) for propylene glycol, 0.19 for PEG 400, 0.27 for glycofurol 75, 0.57 for Cremophor RH40, 0.68 for 2-hydroxypropyl-beta-cyclodextrin and 0.84 for polysorbate 20. The direct effects of the various solvent systems were tested on the human leukaemia cell line K-562 in a standard proliferation assay. Most of the solvents proved toxic with the exception of propylene glycol, PEG 400 and 2-hydroxypropyl-beta-cyclodextrin. Anti-proliferative activity of usnic acid could be demonstrated with an ED50 (amount of substance required to reduce thymidine uptake to 50% of uptake by untreated control culture) of 4.7 microg mL(-1) using PEG 400 and 2-hydroxypropyl-beta-cyclodextrin but only the latter gave satisfactory solubility. 2-Hydroxypropyl-beta-cyclodextrin was thus identified as a solubilizing agent that fulfilled both set criteria of solubility and lack of toxicity against the test cells.
Kristmundsdóttir
Thórdís
T
Faculty of Pharmacy, University of Iceland, Reykjavik, Iceland. thordisk@hi.is
Aradóttir
Halldóra Aesa
HA
Ingólfsdóttir
Kristín
K
Ogmundsdóttir
Helga M
HM
D000704:Analysis of Variance; D001572:Benzofurans / Q000737:chemistry* / Q000494:pharmacology; D002455:Cell Division / Q000187:drug effects; D006801:Humans; D006863:Hydrogen-Ion Concentration; D020014:K562 Cells; D008011:Lichens / Q000737:chemistry*; D012995:Solubility; D012997:Solvents; D013936:Thymidine / Q000378:metabolism
D001572:Benzofurans; D012997:Solvents; C073339:usnic acid; D013936:Thymidine