Table II: Near-infrared spectra comparisons of test product to reference product using ρm and δa.
Table II lists NIR spectra comparison results in terms of the ρm and δacriteria. Essentially, ρm is a ratio-based method, but δa is based on the absolute difference between two spectra (10, 11). Each method compares profiles at the same wavelength and
can include all or selected wavelengths. If the profiles are identical, ρm = 0. Dissimilar profiles result in ρm that approaches infinity. This approach results in a ratio to assess spectra similarity, in which the larger value of either
R t /Tt or Tt/Rt is considered at each wavelength. By virtue of being a ratio approach, ρm normalizes the differences between the profiles against the size of the profiles (i.e., it results in a value that quantifies
the relative difference between the two profiles). δa also considers the relative size of the profiles. The authors used critical limits of ρm > 0.15 and δa > 15 to denote products that were not similar to their references because these limits represent a difference of more than
15% from the reference.
Figure 7. Near-infrared spectra of Norvasc 10 mg test and reference products and authentic generic amlodipine besylate 10
Spectra were compared over the 400–2500-nm range and over the 800–2500-nm range. Because 400–800 nm is the visible range and
color differences (e.g., between dyes) are frequently and easily imparted to products designed for different countries, the
narrower 800–2500-nm range was used to minimize colorant differences.
Figure 8. Near-infrared spectra of Viagra 50 mg test and reference products.
The results of ρm and δa generally followed the results of simple visual inspection of the tablets (see Table II). Most test spectra were similar
to the reference spectra (i.e., ρm < 0.15 and δa < 15), and most products appeared to be authentic. Hence, the authors concluded that test Cialis 20 mg, test Lamisil 250
mg, test Levitra 20 mg, test Norvasc 5 mg, test Viagra 50 mg, and test Viagra 100 mg were authentic. TLC results from the
terbinafine hydrochloride products and API indicated that test Lamisil 250 mg contained the labeled drug. TLC results from
sildenafil citrate products and API indicated that test Viagra products contained the labeled drug.
James E. Polli is a professor at the University of Maryland School of Pharmacy, 20 Penn St., Baltimore, MD 21201. Polli is also a member of Pharmaceutical Technology's editorial advisory board.
Articles by James E. Polli
Stephen W. Hoag
Stephen W. Hoag is an associate professor at the University of Maryland School of Pharmacy, 20 Penn St., Baltimore, MD 21201.
Articles by Stephen W. Hoag