NMR Longitudinal Rotating Frame Relaxation Time (T_1ρ) with a Weak Spin Locking Field as an Approach to Characterize...

Nuclear magnetic resonance (NMR) longitudinal rotating frame relaxation time (T_1ρ), rarely used in low-field NMR, can be more effective than conventional T₁ and T₂ relaxation times to differentiate polymorphic forms of solid pharmaceuticals. This could be attributed to T_1ρ sensibility to structural and molecular dynamics that can be enhanced by changing the strength of the oscillating magnetic field (B) of spinlock pulses. Here, we compared the capacity of T₁, T₂, and T_1ρ to differentiate inactive (A) and active (C) crystalline forms of the World Health Organization essential drug Mebendazole. The results showed that T₁ and T₂ values of both forms were statistically identical at 0.47 T. Conversely, T_1ρ of both forms measured with weak spinlock B fields, ranging from 0.08 to 0.80 mT were statistically different in the same spectrometer. The T_1ρ also has the limit of detection to detect the presence of at least 10% of inactive A form in the active C form. Therefore, T_1ρ, measured with weak spinlock B fields can be an effective, streamlined, and complementary approach for characterizing not only solid active pharmaceutical ingredients but other solid-state materials as well.