Pharmaceutical Assays
Drug screening, drug production, assay of drugs and of impurities, stability tests, uniformity tests and dissolution rate studies are areas where FIA and SIC techniques ffind application, as they often provide the same information as chromatography, yet in a faster and more economical way. Since reagent based assays with UV-VIS, fluorescence or chemiluminescence detection are widely used in pharmaceutical analysis, their automation by FI or microSI techniques was a natural development, as described in numerous publications (See Database and section Resources below). The following examples offer only a glimpse of the many opportunities that versatility of flow based instrumentation offers to pharmaceutical research and quality control.
Drug-protein Binding
Traditionally HPLC is the method of choice for assay of a free drug after its separation (by ultrafiltration or dialysis). If, however, there is a selective reagent available that will combine with the free drug, combination of FI, or even better of microSI, with microdialysis is a preferable choice. The forerunner of such an approach is the FI method for determination of streptomycin/ bovine serum albumin binding constant using chemiluminescence detection (Huang 2001).
Assay of Active Components in Pharmaceutical Formulations
Spectrophotometry, fluorescence and chemiluminescence automated by FI and more recently by SI , as tools for assay of target components has been described in a monograph (Calatayud 1996) and in a recent extensive review (Polasek 2008), as well as in over a thousand research articles. Despite the proven advantages of this methodology, wide acceptance of these modern and efficient techniques has been slow, since the official assay protocols are still based on old fashioned discrete assays.
Automated Drug Dissolution
Since the first work on the dissolution rate of ibuprofene tablets (Liu 1989) the use of flow based analytical techniques, has gained acceptance for this type of continuous monitoring.
Sequential Injection was used to monitor propanolol HCl tablets by exploiting the native fluorescence of propranolol HCl in Krebs–Ringer buffer. The streams from dissolution and permeation modules were sampled with high frequency at three different locations. Conventional SI apparatus served as the central hub that provided connections to detector to auxiliary reagents, and to autosampler which was used to provide standard solutions. (S.A. Motz 2007)
Sequential Injection Chromatography can easily be interfaced with an ongoing process that is to be monitored. In pharmaceutical research it can be a release of an active ingredient from a pill or from a topical pharmaceutical formulation. This approach was proposed and verified (Klimundova 2006) by interfacing an SIC instrument with a Franz diffusion cell, for the study of kinetics of dissolution and permeation of lidocaine and prilocaine released from tablets. Samples were taken in 10 minute intervals over a 10 hour test period, without the need for operator intervention. Since the samples were injected directly from the diffusion cell into the SI instrument, sample volume per analysis could be minimized (10 µL).
The method offers a significant improvement over the generally applied manual technique where periodically withdrawn samples are injected into an LC instrument.
Resources
There are two comprehensive sources of information on the use of FIA techniques in pharmaceutical analysis.
The pioneering monograph “Flow Injection Analysis of Pharmaceuticals” ( J.M. Calatayud, Taylor and Francis, UK, 1997) comprises inter alia a very thorough review of assays based on UV-VIS, fluorimetry, chemiluminescence, atomic absorption and numerous other techniques for a wide variety of active components.
The recent comprehensive review “Pharmaceutical Applications” by M. Polasek in “Advances in Flow Injection Analysis and Related Techniques” ( S.D. Kolev and I.D. McKelvie Elsevier NY. 2008) provides a sweeping overview of opportunities for FI,SI and SIC in the broad area of pharmaceutical research and production.
