Tryptophan, toxic impurity

A 0.1% threshold for identification and isolation of impurities from all new molecular entities is under consideration by the International Conference on Harmonization as an international regulatory standard [4,5]. However, where there is evidence to suggest the presence or formation of toxic impurities, identification should be attempted. An example of this is the 1500 reports of Eosinophilia-Mylagia Syndrome and more than 30 deaths associated with one impurity present in L-tryptophan which were present at the 0.0089% level [6]. 

Gotti et al. [42] reported an analytical study of penicillamine in pharmaceuticals by capillary zone electrophoresis. Dispersions of the drug (0.4 mg/mL for the determination of (/q-penicillaminc in water containing 0.03% of the internal standard, S -met hy I - r-cystei ne, were injected at 5 kPa for 10 seconds into the capillary (48.5 cm x 50 pm i.d., 40 cm to detector). Electrophoresis was carried out at 15 °C and 30 kV, with a pH 2.5 buffer of 50 mM potassium phosphate and detection at 200 rnn. Calibration graphs were linear for 0.2-0.6 pg/mL (detection limit = 90 pM). For a more sensitive determination of penicillamine, or for the separation of its enantiomers, a derivative was prepared. Solutions (0.5 mL, final concentration 20 pg/mL) in 10 mM phosphate buffer (pH 8) were mixed with 1 mL of methanolic 0.015% 1,1 -[ethylidenebis-(sulfonyl)]bis-benzene and, after 2 min, with 0.5 mL of pH 2.5 phosphate buffer. An internal standard (0.03% tryptophan, 0.15 mL) was added and aliquots were injected. With the same pH 2.5 buffer and detection at 220 nm, calibration graphs were linear for 9.3-37.2 pg/mL, with a detection limit of 2.5 pM. For the determination of small amounts of (L)-penicillamine impurity, the final analyte concentration was 75 pg/mL, the pH 2.5 buffer contained 5 mM beta-cyclodextrin and 30 mM (+)-camphor-10-sulfonic acid, with a voltage of 20 kV, and detection at 220 nm. Calibration graphs were linear for 0.5-2% of the toxic (L)-enantiomer, with a detection limit of 0.3%. 

Review of the literature appears to indicate that L-tryptophan itself is generally not toxic in humans, or if it is toxic, very high levels are needed. However, it does appear that under certain conditions, as with certain contaminants or impurities (as in eosinophilia myalgia syndrome), it may contribute to the toxicity of the toxic compounds. 

The occurrence of the eosinophilia myalgia syndrome discussed in Chapter 11 suggests that L-tryptophan containing minute amounts of impurities or contaminants may be causative of a disease state in certain individuals. The role of L-tryptophan itself in this syndrome is not defined. Conceivably, it plays a supportive role to toxic contaminants in selected, susceptible individuals to induce the syndrome. 

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