Insights into Amlodipine Besilate Dissolution Behavior Using UV Imaging

Purpose: In this work a UV imaging method was used to characterize the dissolution behavior of amlodipine besilate solid state forms. Methods: Monohydrate (MH) and dihydrate (DH) forms of amlodipine besilate (AMB) were prepared by recrystallization of anhydrate AMB in water. The amorphous AMB form (AM) was prepared by dehydration of the MH. The samples were compacted by weighing of 6 mg of substance into a steel cylinder (Ø: 2 mm). A torque screwdriver was used to obtain the same compression force for all samples. UV imaging of a dissolution cell (560 μl volume; 3 mm light path) was performed at 280 nm in an area of 9x7 mm. Light microscopy and Raman spectroscopy were utilized before and after UV imaging for solid form identification. Results: The UV imaging technique facilitated the acquisition of spatially and temporarily resolved intrinsic dissolution data of AMB. The AM had a higher dissolution rate than the DH sample. However, Raman spectroscopy showed that the thermodynamically metastable AM sample converted into the monohydrate upon contact with the dissolution media. The high dissolution rate of the AM samples may thus be influenced by an increase in surface area stemming from recrystallization on the compact surface, as well as a higher intrinsic dissolution of the amorphous and MH form. The increase in surface area was showed from both light microscopy and UV imaging. The Raman data displayed that the DH samples did not convert during dissolution. Conclusion: The highly spatially and temporarily resolved data enabled byUV imaging showed a difference between the dissolution rate of AM and DH samples. UV images indicated that AM samples recrystallized during the dissolution experiment, which was verified by Raman. The dissolution rate of the AM samples may have been influenced by the increased surface area generated by this recrystallization.