Determination of organic impurities in pharmaceutical preparations

Pharmaceutical impurities and degradation products: uses and applications of NMR techniques

Current standards and regulations demand the pharmaceutical industry not only to produce highly pure drug substances, but to achieve a thorough understanding of the impurities accompanying their manufactured drug substances and products. These challenges have become important goals of process chemistry and have steadily stimulated the search of impurities after accelerated or forced degradation procedures. As a result, impurity profiling is one of the most attractive, active and relevant fields of modern pharmaceutical analysis. This activity includes the identification, structural elucidation and quantitative determination of impurities and degradation products in bulk drugs and their pharmaceutical formulations. Nuclear magnetic resonance (NMR) spectroscopy has evolved into an irreplaceable approach for pharmaceutical quality assessment, currently playing a critical role in unequivocal structure identification as well as structural confirmation (qualitative detection), enabling the understanding of the underlying mechanisms of the formation of process and/or degradation impurities. NMR is able to provide qualitative information without the need of standards of the unknown compounds and multiple components can be quantified in a complex sample without previous separation. When coupled to separative techniques, the resulting hyphenated methodologies enhance the analytical power of this spectroscopy to previously unknown levels. As a result, and by enabling the implementation of rational decisions regarding the identity and level of impurities, NMR contributes to the goal of making better and safer medicines. Herein are discussed the applications of NMR spectroscopy and its hyphenated derivate techniques to the study of a wide range pharmaceutical impurities. Details on the advantages and disadvantages of the methodology and well as specific challenges with regards to the different analytical problems are also presented.

Synthesis of 9,9'-[1,2-ethanediylbis(oxymethylene)]bis-2-amino-1,9-dihydro-6H-purin-6-one, an impurity of acyclovir

The synthesis of 9,9'-[1,2-ethanediylbis(oxymethylene)]bis-2-amino-1,9-dihydro-6H-purin-6-one, a minor impurity of acyclovir, is described. Starting with commercial N-(9-acetyl-6-oxo-1H-purin-2-yl)acetamide, the process uses an acid catalysed phase transfer catalysis (PTC) process to produce the selective alkylation at the 9 position of the guanine ring.

Drug safety, drug quality, drug analysis

Controlling and minimizing the side effects of drugs are the key issues in assuring the safety of drug therapy. Since side effects are inherent properties of the drug material, these cannot be influenced by drug analysts. At the same time drug analysts play a predominant role in assuring the quality of bulk drug materials and drug formulations and this is also closely related to the safety issue. The three main attributes of drug quality are identity, strength and purity. Of these, in the case of bulk drug materials, purity is of prominent importance: by the identification (structure elucidation) and quantitative determination of the impurities and degradation products, the risk of their contribution to the side effect profile of the drug materials can be avoided or at least controlled/minimized. The development in the field of chromatographic and spectroscopic methods in the last decades has led to changes in the philosophy, structure and requirements in the monographs of drug materials in the principal pharmacopoeias. Although the approaches of the European and US Pharmacopoeias are somewhat different, a common feature is the shift of focal point toward purity tests. In contrast to this, relatively few changes are observable in the field of the assay methods for bulk drug materials: non-selective titrimetric and spectrophotometric methods are still widely used. Since the results of these do not contribute to the safety issue, the omission of these tests and substitution by the "mass balance" concept is recommended. The effectiveness of the tendency of replacing non-selective methods by selective ones (mainly HPLC) is also questionable. The reason for this is that due to the limited precision of the HPLC assay the drug content obtained by the mass balance concept is a much better quality control attribute for bulk drug materials than that obtained by HPLC. It is recommended that classical assay methods (including HPLC) be used in exceptional cases only and the time and energy thus spared be used for more important impurity-related issues that directly contribute to the safety of drug therapy.