Does an infallible method exist? If we are realistic, we have to say no. Different methods have different uses, different strengths and different weaknesses. It makes sense that to increase the stringency of any quality control system, using a combination of methods creates a level of redundancy that can allow manufacturers to feel secure in the knowledge that their tests are working perfectly. This is especially important in the case of pharmaceutical specials manufacturing where bespoke medicine formulations are made for sick patients for whom licensed medication is not appropriate.
HPLC: The gold standard in specials manufacture For many chemical analysis processes, HPLC is the gold standard technique and specials manufacture is no different. Over decades of use, HPLC has demonstrated time and again that it is the technique of choice for validation and quality control, and for good reason.
HPLC works. Specials manufacturers need to know two key pieces of information:
Is what goes into the formulation what I think it is?
Is the end product exactly what I expect?
HPLC can answer both of these questions with a high degree of accuracy which has helped cement the status as gold standard. It is also relatively fast, high resolution and versatile when compared to other chromatography methods like gas chromatography and TLC.
The output from HPLC analysis are so ubiquitous in manufacturing that they are quick to interpret for anyone in the field. The maturity of the method also has a phycological effect on manufacturers and customers alike. Knowing that HPLC has been used in validation gives an extra level of confidence.
Where does HPLC fall down? For all the benefits of HPLC, there are three major downsides—complexity, cost and speed. HPLC is a complex method that requires extensive training to both operate the equipment and analyse the results. Without good training, HPLC will be impossible to use or there is a greater risk that human error with be introduced into the quality control process.
The second major drawback is the cost. HPLC instruments are expensive to purchase, maintain and operate. Expensive reagents are needed for each HPLC run and highly trained staff are needed to generate reliable data. Costs can be managed under some circumstances, but due to the high level of accuracy and precision required during specials manufacture, there is no way around extensive testing. This means that costs can quickly balloon which is not an insignificant problem for specials manufacture, especially when batches can be quite small.
The most controversial drawback of HPLC is the speed. In the past, HPLC was hailed as a relatively speedy method. Compared to mass spectrometry and some other chromatography methods, HPLC was faster, especially in the hands of an experienced operator. But speed is relative to what other methods are available and also what processes need to be measured. During specials manufacture, batches of medication are often held until testing has confirmed the purity and concentration of the end product. Time spent doing this is time that the medication could be used to treat patients. This delay is compounded in many facilities that don’t have direct access to an HPLC instrument and have to send samples away for independent analysis. In this case, the delay for product release can be days.
Things to consider when choosing complementary methods When moving away from the security provided by a gold standard technique like HPLC, the choices can be overwhelming, and it is easy to be sceptical. However, there is an easy way to filter the choices to make sure that the method will be suitable. Essentially, the new method has to be similar, but different.
Similar to HPLC If the new method cannot be compared to the gold standard then it will be almost impossible to validate. New methods should have a relatively straight forward way of comparing the results to HPLC. This is crucial in the setup phase where comparing results from the two methods will allow for accurate benchmarking and standarisation. This will also be very important when both methods are ultimately implemented to easily spot when problems have occurred.
But different from HPLC Having two very similar methods is not an optimal solution. The perfect approach is to find a method that provides similar results, while filling in some of the drawbacks of HPLC. For specials manufacturing, any new method should be cheap to setup and run with a low cost per sample. Ideally, it should also be quick and simple to perform.
What other methods are available? When considering everything that would make a good complementary method to HPLC, UV absorption spectroscopy stands out. While the technique is quite broad, there are a number of features that make it an attractive option to use for quality control of specials. Firstly, it is much faster and cheaper than HPLC, as sample require almost no specific preparation. The equipment is also cheaper and requires very little training to use. The small footprint of most spectroscopy instruments means that it is much easier to have them close to where samples need to be measured.
Of course, this is not enough to introduce a new method into a testing procedure. Any new method has to give comparable results. When properly calibrated, some spectroscopy methods are capable of identifying different chemical species within a solution as well as accurately determining the concentration. This means that UV absorption spectroscopy fits all the criteria to be introduced as a new quality control method.
How to implement a new method When implementing a new quality control method, it is worth taking the time to set it up properly. It is important to know, that when taking a spectroscopy measurement and an HPLC measurement they will probably be similar, but good optimisation can mean that results will be almost identical.
In order to do this, it is essential to run both methods in parallel at the very beginning, while recording all the results in order to understand where calibration is needed. It is essential at this point to use samples from real batches. A common error is to standarise new equipment on specially prepared laboratory standards. This will allow the instruments to be quite accurate, but in order to go to the next level of accuracy, always calibrate and test against samples from batches, which will always vary slightly from formulations prepared in the laboratory.
Once both methods have been standarised, it is important to continue to run them in parallel for some time. This will help give confidence that there are no variations or discrepancies between results from the HPLC and spectroscopy. Once this has been satisfied, the methods no longer need to be run in parallel, after all, one purpose of introducing spectroscopy is to bring cost and time down. Instead of running both tests in parallel, each test should be used at different points in the manufacturing process and occasionally the same sample should be tested using both methods to provided an extra level of security. This level of care and attention to detail will delight pharmacists and help to show regulators that you are being proactive about quality assurance.
Getting the best of each method In order to get the best out of each method it’s important to carefully assess where they can be used and what information they will provide during the manufacturing process. For example HPLC will continue to be the gold standard and regularly including HPLC measurements in testing protocols is important. However, there are times when HPLC is too slow or impractical, for example it would not be possible to quickly test a formulation in the middle of a batch. In these cases, a faster method like UV absorption spectroscopy would provide additional information and security about the quality of a batch. Routinely using both methods and comparing them means that after a while, they will be able to be used interchangeably during the manufacturing process meaning that when specials need to be released quickly, a simple spectroscopy assay will provide all of the information needed.