Metal Contamination in Biopharmaceutical Drugs: Solving a puzzle without all the pieces
Mary Stellmack and Dr. Kent Rhodes, McCrone Associates Inc., Westmont, Illinois
Contract Pharma - October 1, 2010
Metal particles are common contaminants in the drug manufacturing process, a problem that should be addressed before the product goes to market. However, the recent recalls of several Rx and OTC medications due to metal contamination show that more needs to be done to prevent contaminants from reaching the public. Though small in size, repeated ingestion of metal-contaminated pharmaceuticals can lead to metal poisoning — lead and chromium particles being the most dangerous.
Despite stringent quality control (QC) standards regulated by the FDA and highly trained in-house QC inspectors, microscopic metal contaminants as well as other defects and particulates that are in the sub-visible size range (< 50μm) continue to go unnoticed. Many biopharmaceutical organizations seek the expertise of independent analytical or microanalysis laboratories that have the experience, skill and necessary instrumentation to analyze microscopic metal particles. Several current techniques and analytical methods are available to detect, isolate, and identify such impurities. These laboratories have discovered that despite their advances, the chemical nature of metal corrosion makes it impossible at times to identify the source of metal contamination.
Isolating the Contaminant
Solid and liquid pharmaceuticals are typically examined and prepared for analysis in a cleanroom to avoid contamination of the samples. They are first examined with the naked eye, then using a stereomicroscope. The most common physical signs of a metal contaminant are the appearance of shiny metal flakes, or dark, brittle particles ranging in color from red or orange to brown or black.
If visible particles are present in a liquid sample, a magnet can be drawn along the vial wall to collect susceptible particulate. If the particles follow the movement of the magnet, the liquid sample likely contains metal particles. However, the absence of a response to the magnet does not eliminate the possibility of metal contamination. The liquid sample can be filtered on a polycarbonate membrane filter, typically 0.2 µm or 0.4 µm pore size, in a vacuum-filtration apparatus. The smooth, shiny surface of such filters allows an analyst to see the microscopic metal particles (Figure 1) and remove them for analysis with a bit of adhesive on a tungsten needle.
Metal particles in solid tablets may appear as discrete masses that can be removed in one piece from the tablet. If lodged at the tablet surface, particles as small as several hundred micrometers can often simply be removed with forceps for analysis. Smaller discrete metal particles may need to be freed from their surroundings by applying a few micro-drops of water or other suitable solvent to the tablet surface. This softens or dissolves the surrounding tablet material, freeing the metal particle, which can then be lifted with a bit of adhesive on a tungsten needle.
In some cases, solid tablets exhibit gray or brown stains that only upon further examination provide evidence of metal contamination (Figure 2). These stained areas commonly contain sub-visible metal or metal corrosion particles on the order of 10 µm and smaller, mixed with normal tablet materials and sometimes machine oil. A tungsten needle is used to remove and transfer a portion of the stained material to a glass slide. A micro-drop of hexane or other suitable solvent is applied to the stained material, and any oils present are extracted and identified by infrared spectroscopy. The remaining insoluble materials, which include the metal particles and tablet materials, are divided into two portions. One portion is analyzed by infrared spectroscopy to verify the presence of the normal tablet materials, and the other portion is submitted for analysis by scanning electron microscopy (SEM) with an energy dispersive x-ray spectrometer (EDS) analysis to confirm the presence of metal or metal corrosion particles.
Using SEM/EDS To Positively Identify the Contaminant
The SEM uses electrons instead of light to form an image. The sample is bombarded with electrons, and atoms in the sample interact with the electrons to produce secondary electrons and backscattered electrons. These electrons are collected by a detector and used to produce a high-quality morphological image showing the physical features of a sample. Metal particles can be easily distinguished from organic materials by their backscattered electron signal.
The electron beam of the SEM also generates x-rays from the sample. Each element has a unique x-ray pattern, and the EDS detector is used to collect the x-rays and analyze their energies. For a pure metal particle, such as iron or aluminum, the EDS spectrum will display a composition showing close to 100% iron or aluminum, respectively. If the particle is somewhat oxidized or corroded, the EDS spectrum will display the presence of oxygen in some amount, and a proportionately lower amount of the metal. For example, a sample of oxidized iron might produce an EDS spectrum with 70 wt% iron and 30 wt% oxygen. For “real world” samples, metal corrosion particles often display small amounts of multiple elements, since the source metals are typically alloys and not pure metals, and may have been exposed to various environments prior to analysis.
Solving the Puzzle
The most common source of metal contaminants in biopharmaceutical products are wear particles, generated from processing machinery. The most common type of metal found in products such as liquid injectables or over-the-counter pharmaceuticals is stainless steel.
There are more than 150 grades of commercially available stainless steel alloys, each distinguished by the composition of alloying elements. The EDS data from a metal particle can be compared to published databases of common stainless steel compositions. In some cases, possible sources for the particle can be narrowed down to a few likely choices, and might be traced to the type of steel used in certain pieces of manufacturing equipment.
Unfortunately, finding a solution is not always that easy. The majority of metal particles that are isolated from biopharmaceutical products are often slightly corroded or oxidized due to exposure to air, water, bacteria, or chemical exposure such as high chloride or sulfate environments. During the corrosion process, the ratios of iron and alloying elements may change, and the amount of oxygen in the metal sample increases. This changes the elemental composition to the extent that many times it is not possible to match the particle to any known source (Figures 3 and 4). However, clues about the origin of the particles can sometimes be obtained by the presence or absence of certain alloying elements. For example, molybdenum does not exist in all types of stainless steel. The presence or absence of this element narrows down the list of likely sources.
The natural wear and tear of manufacturing machinery is a facet of production that cannot be avoided. Biopharmaceutical companies need to continue to diligently monitor not only the quality of the product but also the condition of the manufacturing machinery.
Working in conjunction with independent analytical laboratories will help decrease the incidence of metal-contaminated pharmaceuticals from reaching pharmacy or store shelves and ultimately the consumer.
Mary Stellmack is a senior research chemist and Dr. Kent Rhodes is a senior research scientist at McCrone Associates, Inc. (http://www.mccroneassociates.com). Both are also instructors at Hooke College of Applied Sciences, the education division of The McCrone Group. The college can be found at http://www.hookecollege.com