Minocycline is the treatment of choice for acne vulgaris and has been shown to cause discoloration of the natural tooth in adults.    Discoloration has been reported to be the product of an oxidation reaction. Laboratory studies have shown that discoloration can be induced by exposure to light (Quintessence International, 37 (8) 637-646), and that an antioxidant can block its formation. (J Esthet Dent, 10(4):182-6).

Objectives:  The purpose of this study was to evaluate the discoloration of minocycline in vitro and to determine subsequent changes after the addition of antioxidants.

Methods:  Pure minocycline dissolved in distilled water (positive control) at the concentration 4mg/mL and was used against solutions with minocylcine and Epigallocatechin gallate (EGCG) and one with N-acetyl cysteine (NAC).  Distilled water was used as the negative control.  EGCG and NAC are antioxidants derived from green tea and glutathione respectively.  HCl was added to each solution until pH 2 to aid in solubility.  The solutions were placed under a xenon lamp and the absorbance of each solution was measured by a spectrophotometer (Thermo MAX micro plate reader) at wavelength 414nm everyday for 5 days and then at day 10 to monitor the discoloration.  Experiments were done in multiple sets and in triplicates.  Digital images of well plates were recorded.  Color change from the baselines and corresponding time periods were calculated and graphed from the spectral data. 

Results: Example of one trial and plate showing color changes.

	Day
Treatment	1	2	3	4	5	10
Distilled Water	0.043 (0.001)	0.044(0.0015)	0.045 (0.0053)	0.043 (0.0012)	0.042 (0.001)	0.038 (0.002)
Minocycline	0.57 (0.03)	0.7 (0.05)	0.81(0.06)	0.88 (0.13)	1.05 (0.145)	2.4 (0.55)
Mino/EGCG	0.48 (0.004)	0.6 (0.016)	0.69 (0.022)	0.76 (0.04)	0.86 (0.06)	1.9 (0.15)
Mino/NAC	0.51 (0.014)	0.63 (0.018)	0.76 (0.028)	0.84 (0.05)	0.95 (0.052)	2.14 (0.133)

Conclusion:  EGCG and NAC appeared to be effective in preventing discoloration of minocycline in vitro after day 5. 

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