Stable isotopes exist naturally as mass variants of chemical elements. Carbon-13, for instance, consisting of seven neutrons and six protons, accounts for one percent of all carbon. The variation in the 13C/12Cand other isotopicratios of pharmaceutical components can be used to trace the isotopic provenance of fingerprint of APIs or drug products via isotope-ratio mass spectrometry.
|John P. Jasper|
PharmaManufacturing.com talked with Jasper about what the future holds for Multiple Stable-Isotopic Analysis (MSIA) as a means of process analysis.
Note: For our discussion with Jasper on MSIA as a forensic tool for product authentication, click here.
PM: Youre looking into isotopic analysis as a means of understanding pharmaceutical processes. What got you started along this path?
JPJ: Stable-isotope chemists have known for decades that the observed isotopic composition of materials is a function of two variables:
- the stable-isotopic composition of its raw materials (or reagents)
- the synthetic processes used to produce them.
After a few years of presenting the stable-isotopic characterization of pharmaceutical materials and these principles, pharmaceutical scientists became curious about the mechanisms that determine the isotopic composition of these materials. Mr. Brian Duffy and Dr. Larry Weaner, both from Johnson & Johnson, came to us with a suite of 53 samples of the antiepileptic drug, Topiramate, that were produced by three separate synthetic pathways. They wanted to know if we could differentiate how the samples were manufactured based on their isotopic compositions. In this preliminary study of archive samples, the stable-isotopic composition of the starting raw materials was unknown, but, despite that, we observed distinct clustering of data depending on the synthetic pathway used.
We are now in the midst of a wholly-controlled (i.e., reagents and processes) study to differentiate the effects of these two variables on the observed composition of pharmaceutical materials. In essence, our research has moved back upstream, from the isotopic composition of the final product to the isotopic composition of the materials and the processes occurring within the manufacturing plant. (For more detailed information on these projects, please visit www.molecularisotopes.com/pdfs.html)
PM: Tell us a little more about Johnson & Johnsons interest in isotopes as a means of process analysis.
JPJ: At this point, they have a scientific interest to look beyond the uncontrolled observational results to determine if it is possible to differentiate the processes that caused them.
PM: Explain, briefly, what you mean by isotope fractionation, and its applications.
JPJ: Light and heavy isotopes are commonly distributed unevenly among reactants and in any chemical reaction there may be differences in the reaction rates between these isotopes because heavier atoms form stronger, more stable bonds. In cases where chemical reactions do not proceed to completion or when multiple products are formed, isotopic fractionation may occur due to different isotopic reaction rates, which result in an uneven isotopic distribution in the product(s) in favor of the isotope having the faster reaction rate.
In principle, the isotopic compositions of chemical products can be predicted from the isotopic compositions of the starting materials together with knowledge of the fractionations. The latter can, however, be predicted quantitatively only when complete mass balances are available and when the kinetic and equilibrium isotope effects associated with all relevant chemical reactions are known accurately. Related calculations are discussed by Hayes (2004, www.nosams.whoi.edu/docs/IsoCalcs.pdf).
While calculations do provide a means of estimating ranges of variation, the final isotopic compositions of products can be measured precisely rather than predicted. In future work, we will expand the scope of analyses so that the power of multiple stable isotopic tracing can be adjudged in detail.
The utility of knowing synthetic-isotope fractionations is that they are a quantitative stable-isotopic process monitor that integrates certain reaction variables that contribute to the isotopic fingerprint of a given product. Noting that the isotopic composition of reagents can be measured as those of pharmaceutical products and, if one knows the isotopic fractionation of a given reaction, one can predict the isotopic composition of the products. Once one knows the isotopic composition of the reagents, if the observed isotopic compositions of products are not as predicted, then one can conclude that some variable in the production scheme was altered. It could have been reaction rate or some variable such as pressure, temperature, or reagent abundance that contributed to the rate. Isotopic composition integrates these reaction variables and therefore can be used to monitor them.
PM: What is the future potential of stable-isotopic analysis as a means of process analysis?
JPJ: Since the stable-isotopic composition of products integrates both the isotopic composition of the reagents and of the process, such measurements are overall integrators of the process of production. In many circumstances, one can determine the key variable that is causing isotopic variations in the product. So, isotope measurements can be used to monitor the production process.
In addition to synthetic fractionation, stable isotopes have other plausible applications in pharmaceutical manufacturing. One of the main applications of stable-isotope chemistry in the earth sciences is monitoring the mixing of components; measurement of the isotopic composition of mixing end members and mixed products should give a very precise measure of the composition of the mixture. Since isotopes typically fractionate during crystallization, isotopic composition could also be used to monitor the extent of crystallization, a common process in chemical and pharmaceutical manufacturing.