1. The authors fail to distinguish adequately between
photodegradation, in which bilirubin is degraded to substances of lower
molecular weight, and photoisomerization, in which the pigment’s
structure changes but no degradation occurs. Both processes contribute
to the effects of phototherapy in humans, but their relative
contributions are presently unknown. However, in the Gun rat animal
model photoisomerization is more important than photodegradation and the
same is likely to hold for humans. In the article the authors focus
largely on photodegradation, the pathway of least importance.
2. The relative effectiveness of the different lights
tested was determined by comparing their effects on methanolic solutions
of bilirubin in vitro. Residual bilirubin was measured after
15-120 min of exposure, by which time a substantial fraction of the
original bilirubin had been bleached and degraded. Preparation of the
methanolic solutions was not described. Aside from the fact that
bilirubin is insoluble in methanol, the photochemistry of bilirubin in
organic solvents is rather different from that in serum or aqueous
albumin solutions. Therefore, methanolic solutions of bilirubin are
inappropriate for comparative testing of phototherapy lights.
3. The test solutions were clearly over-irradiated
and not sampled early enough to detect the relative rates of formation
of the physiologically important photoisomers of bilirubin. Bleaching of
bilirubin has been used in previous studies of the relative efficacy of
phototherapy lights, but there is no evidence that this is a valid
method, especially when bleaching is allowed to continue to a point
where secondary reactions of no relevance to phototherapy may be taking
place. Measuring initial rates of formation of bilirubin photoisomers
would probably provide a more valid and clinically relevant method.
4. The authors claim to have developed a novel high
precision HPLC/MS technique for measuring bilirubin photoisomers, yet
presented no examples of typical separations. Samples were extracted
with a solvent containing formic acid. Configurational photoisomers of
bilirubin, the most rapidly formed isomers in humans, are highly
sensitive to acids, undergoing reversion to unisomerized bilirubin.
Therefore, it seems unlikely that these important photoproducts would
have survived the HPLC conditions. Reference standards were prepared by
irradiating bilirubin in methanolic solution, but no details were
provided of how specific photoisomers or other photoproducts were
identified and characterized.
1. Subramanian S, Sankar MJ, Deorari AK, Velpandian T*,
Kannan P, Prakash GV, et al. Evaluation of phototherapy devices
used for neonatal hyperbilirubinemia. Indian Pediatr. 2010; Nov 30 (E-pub
ahead of print).