Dissolution properties of cypermethrin/cyclodextrin complexes

Cypermethrin-a very effective pyrethroid-type insecticide-has been complexed with beta-cyclodextrin and peracetylated-beta-cyclodextrin with different guest content. Dissolution measurements by reversed phase HPLC method, together with UV-spectrophotometry, differential scanning calorimetry and thermogravimetry were applied to prove the inclusion complex formation and characterize the complexes. With the help of the thermal analysis the really complexed (strongly bound) and surface-bound guests were distinguished. All of the beta-cyclodextrin complexes show better dissolution rate than the pure guest. In case of inclusion complexes an oversaturated solution was formed with extremely high concentration of active substance (6-19 mg L-1) during the first couple of minutes then the concentration decreased gradually until it reached the equilibrium solubility value of the complex (2 mg L-1). The cypermethrin/peracetylated-beta-cyclodextrin complexes prepared with organic solvent method showed slightly retarded dissolution profile compared to the pure guest. The area under the dissolution curves was introduced for quantitative characterization of the dissolution rate. The release was found to depend on the complexed guest content of the samples. The continuous variation plots used first for this parameter gave information on the stoichiometry of the complexes: 1:2 cypermethrin/beta-cyclodextrin and 1:1.25 cypermethrin/peracetylated-beta-cyclodextrin.