Although drug discovery gets much of the attention in the pharmaceutical industry, stability plays an equally fundamental role. A drug must remain potent and safe long after it leaves a manufacturing facility. Ensuring this safety requires a range of technologies and applications.
The complexity of stability testing arises from the many factors that must be considered. For example, such tests examine the active pharmaceutical ingredient, other components of the formulation, and even interactions with packaging and other drugs. “The active pharmaceutical ingredient may be very stable, but maybe free radicals in an excipient in the formulation initiate degradation,” says Richard Ladd, PhD, senior director, pharmaceutical and life sciences marketing at Waters (Milford, Mass.). “Also, the tests must check to see if anything leaks out of the drug package—like a bottle or blister pack—and into the product or vice versa.” Overall, stability tests must confirm that a drug remains active—meaning that its structure doesn’t change over time—and it must not create toxic degradation products.
Different kinds of stability testing also take place at various stages of development. “Quite often you remove compounds from development because they are too unstable to be commercially viable,” says Ladd. “So early on you do rapid testing to predict the stability and pick winners.”
Stages of stability testing
The levels of stability testing tend to come in three categories: forced, accelerated, and real-time stability. In forced-stability testing, a company exposes a compound to a harsh environment, such as high temperature or humidity. The temperature often verges on the melting temperature of the active pharmaceutical ingredient. This can also include testing under intense light. The results can be used to make predictions about reaction kinetics.
Accelerated-stability testing is less harsh. It can include temperatures of, say, 50 C, to predict what might happen to a drug at room temperature over an extended period of time.
For real-time stability testing, pharmaceutical companies store a drug at room temperature, under natural light, and expected levels of humidity in the areas where the drug will be sold. These tests often run for years. In addition, the real-time testing must include potentially damaging scenarios. “It covers if someone leaves the medication on the dashboard in the car and then goes to the beach for the day or if drugs being shipped around the world get left out on a pallet at an airport,” says Ladd.
Components from chromatography
To identify components before and after long-term storage, a drug researcher often starts by separating them with liquid chromatography (LC). “Sometimes you can use LC, or LC/MS [mass spectrometry], dissolve the drug while making sure it all goes into solution, and then separating the components to see if you get the same data before and after long-term storage,” says Kevin Menard, PhD, business manager, materials characterization at PerkinElmer (Waltham, Mass.).
The Acquity UPC2 from Waters provides a new way to separate components. “It’s like taking liquid and gas chromatography and making a new separation platform,” Ladd explains. This can even separate chiral components, and Ladd says, “80% of drugs in development are chiral, so we have to understand the chirality of degradation.”
Michael Frank, marketing manager for analytical HPLC (high-performance liquid chromatography) at Agilent (Santa Clara, Calif.), says, “There’s tendency for our customers to move to ultra-HPLC [UHPLC] systems. It provides fast and high resolution analysis.” He adds, for identification of degradation products, mass spectrometry—using an instrument such as the Agilent 6550 Quadrupole time-of-flight LC/MS—becomes very important.
When looking for degradation products, Frank points out that “in spite of the power of modern MS systems, LC still is very important. Since you don’t know what you’re looking for, you don’t know how the products will elute from the column.” Some of the degradation products, for example, might arise from small transformations of the chemical structure of the active ingredient, and those could be difficult to separate.
One-dimensional LC often can’t reveal all impurities. “Even worse, isobaric chemical rearrangement products cannot be distinguished at all by MS,” says Frank, “and that’s hard to separate with one-dimensional LC, but you can very often with two-dimensional LC.” In such situations, researchers can use the Agilent 1290 Infinity 2D-LC. This system arises from Agilent’s 1290 Infinity Binary LC system and a new two-dimensional LC valve. It includes software that helps a researcher set up a two-dimensional LC method in just minutes.
When asked how accurate existing tests of degradation are over time, Jennie Wang, PhD, program director in the biosciences division at SRI International (Menlo Park, Calif.), says, “HPLC testing should be accurate as long as they are validated for stability-indicating assays.”
Counting on calorimetry
Differential scaling calorimetry (DSC) can also assess the efficacy of a stored drug. A drug’s melting point before and after storage can determine if it includes impurities. Menard also points out the rising concern with photo-induced damage. He describes work using PerkinElmer’s DSC 8500 in which samples were irradiated with high levels of ultraviolet (UV) light—roughly 100 times the UV intensity at noon in some cases—for a short period and then tested for changes in melting point. “Some compounds fall apart after UV,” says Menard, “and some lay there forever and just look at you.”
Menard notes that the DSC 8500 is a double furnace designed for accelerated heating rates. “It can go as high as 750 [C per] minute,” Menard says. “When you heat that fast, you can trap polymorphic forms so you get the initial conditions in the sample.”
Regulatory agencies show an increasing interest in photo-stability, Menard says. “A vaccine shipping to, say, Africa might not go to a place with good refrigeration or storage.” So the vaccine’s photo-stability can be crucial. “Even some household drugs should not be stored exposed to sunlight, because they will degrade over time,” Menard says.
In some cases, the concern arises from specific components. For example, Menard says, “Many tablets include lactose, because it’s a good packing agent, and regulatory agencies want to know how that behaves under humid conditions.” That can be tracked with PerkinElmer’s DMA 8000, which is a dynamic mechanical analyzer and humidity control system.
As already mentioned, troublesome components can also come from packaging. “If you store a drug in a bottle,” says Menard, “you can use our TL-9000 TGA-FTIR-GCMS system to see what might have come out from the container in very small amounts.”
Never underestimate the value of stability
Ultimately, stability determines a drug’s marketability. Based on more than two decades working in the pharmaceutical industry, Ladd says, “I’ve worked on oncology drugs that were shown to be very efficacious in the clinic, but they were removed from development because of stability reasons.”
The technologies used to ensure long-term efficacy and safety help distinguish promising new drugs from failures.
About the Author
Mike May is a publishing consultant for science and technology based in Austin, Texas.