We read with great interest the article by Brusch et al. [1], in which the authors report two patients with primary allergic reaction to pholcodine who were subsequently found by skin and serological testing to have evidence of sensitization to a neuromuscular blocking agent. We fully agree with Brusch et al. about the necessity to perform a more complete study addressing the link between pholcodine and neuromuscular blocking agent anaphylaxis. However, we have a better opinion about the use of pholcodine skin testing, with less fear about nonspecific histamine release, because the test is performed by allergologists with diluted syrups (corresponding to 0.1-0.01 mg ml −1 of pholcodine) according to our experience and to the published data [2, 3]. We therefore decided to assess the stability of a pholcodine (pharmaceutical grade) 1.0 mg ml −1 solution over a 14-day period in order to be able to propose to allergologists a solution ready to dilute for standardized skin testing.The European Pharmacopoeia monograph on pholcodine mentions the following synthesis or degradation impurities: morphine (synthesis and degradation, impurity A), codeine (derived from morphine, impurity B) and impurity D resulting from a double substitution by morpholinoethyl radical (Figure 1) [4]. Codeine and morphine are known to be responsible for nonspecific histamine release. Other detectable impurities are oxidation derivatives (N-oxide, N′-oxide and N,N′-oxide).Preparation was performed by our pharmacy preparation unit. Five hundred milligrams of pholcodine monohydrate powder (batch 22042A004; PCAS, Longjumeau, France) were dissolved in 100 ml of sterile NaCl 0.9% (Aguettant, France) and made up to 500.0 ml, filtered through 0.22 μm cellulose acetate filter and divided into 5 ml fractions in a series of 14 glass vials. Seven of the vials were kept at 2-8°C, while the other seven were left at room temperature (20-25°C); all were protected from light. All analyses were performed for two samples on days 0, 1, 2, 3, 6 and 14.As recommended by the European Pharmacopoeia, liquid chromatography was chosen as the primary analysis technique. High-performance liquid chromatography was carried out with an Elite LaChrom® chromatographic system (VWR, Neuilly sur Marne, France) composed of a quaternary pump, a diode array detector and an injector, coupled to an EZChrom Elite™ Software Chromatography Data System. The column was a Luna® Phenomenex (Le Pecq, France) composed of 3 μm spherical particles of end-capped phenylhexylsilyl silica gel (column: 7.5 cm length, 4.6 mm diameter, 10 nm pore diameter, 400 m 2 g −1 surface). The working temperature was 35°C. The detection was set at 238 nm. The mobile phase was prepared daily by adding 75 ml of acetonitrile to 50 ml of tetrahydrofuran for chromatography, made up to 1000 ml with 0.02 M phosphate buffer solution and adjusted to pH 7.90 ± 0.05 with 0.2 M sodium hydroxide; the pH should not exceed 8.0. The limit of quantification (signal/noise ratio = 10) for impurities A, B and D was close to 0.004%. ...
