Figure 9. Near-infrared spectra of Viagra 100 mg test and reference products.
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NIR comparison also agreed with simple visual inspection for test Cialis 50 mg (ρm = 0.303 and δa = 21.1 over the 400–2500-nm range). Disintegration testing of test Cialis 50 mg further supported the thesis that the product
was counterfeit. Three individual tablets each required more than 15 min (15:20, 16:34, and 17:35 min) to disintegrate. Reference
Cialis 20 mg disintegrated in less than 5 min (3:57, 3:57, and 4:10 min for each of three tablets).
Test Norvasc 10 mg was the exceptional product. Simple visual inspection indicated that the product was authentic, but NIR
could not determine its authenticity (ρm = 1.07 and δa = 54.4 over the 400–2500-nm range). TLC results from the amlodipine besylate products and API indicated that the test Norvasc
10 mg product contained the labeled drug.
Comparison of the authentic generic amlodipine besylate 10 mg to the reference.
Figure 7 plots the NIR spectra of authentic generic amlodipine besylate 10 mg. Like test Norvasc 10 mg, generic amlodipine
besylate 10 mg exhibited a spectrum that was different from that of the reference (i.e., ρm > 0.15 and δa > 15). Generic amlodipine besylate 10 mg was manufactured by Mylan Laboratories. Comparing the spectral differences of the
Mylan product and the test Norvasc 10 mg product with their references showed that the Mylan product differed from the reference
in a pattern similar to the way the Hong Kong product differed from its reference. Each showed a baseline shift, but provided
similar peak positions to references. In spite of the larger ρm and δa values for generic amlodipine besylate 10 mg and test Norvasc 10 mg, the generic product was presumed authentic, and test
Norvasc 10 mg could be authentic. TLC results showed a single spot from test and reference Norvasc 10 mg products, as well
as from the Mylan product, that agreed with the TLC results from amlodipine besylate API.
Conclusion
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 and test Cialis 50 mg and test Norvasc 10 mg were potentially counterfeit. Generic amlodipine
besylate 10 mg did not provide a spectrum that could be described as similar to that of the reference product, although no
evidence supports the thesis that this product is counterfeit.
These results indicated that NIR is a viable approach for the quick assessment of potentially counterfeit products, particularly
because NIR spectra are sensitive to both composition and physical properties. Results also indicated that initial field testing
for counterfeits can be accomplished with a handheld NIR spectrophotometer. To minimize incorrect NIR-based conclusions about
products, NIR spectra should be interpreted with care.
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