Bending the Curve

By Dr. Scott Rudge

To understand the best ways to develop preparative and industrial scale adsorption separations in biotechnology, it’s critical to understand the thermodynamics of solute binding. In this blog, I’ll review some basics of the Langmuir binding isotherm. This isotherm is a fairly simplistic view of adsorption and desorption, however, it applies fairly well to typical protein separations, such as ion exchange and affinity chromatography.


A chemical solution that is brought into contact with a resin that has binding sites for that chemical will partition between the solution phase and the resin phase. The partitioning will be driven by some form of affinity or equilibrium, that can be considered fairly constant at constant solution phase conditions. By “solution phase conditions”, I mean temperature, pH, conductivity, salt and other modifier concentrations. Changing these conditions changes the equilibrium partitioning. If we represent the molecule in solution by “c” and the same molecule adsorbed to the resin by “q”, then the simple mathematical relationship is:

If the capacity of the resin for the chemical is unlimited, then this is the end of the story, the equilibrium is “linear” and the behavior of the adsorption is easy to understand as the dispersion is completely mass transfer controlled. A example of this is size exclusion chromatography, where the resin has no affinity for the chemical, it simply excludes solutes larger than the pore or polymer mesh length. For resins where there are discrete “sites” to which the chemical might bind, or a finite “surface” of some kind with which the chemical has some interaction, then the equilibrium is described by:

and the maximum capacity of the resin has to be accounted for with a “site” balance, such as shown below:

Where S_tot represents the total number of binding sites, and S₀ represents the number of binding sites not occupied by the chemical of interest. The math becomes a little more complicated when you worry about what might be occupying that site, or if you want to know what happens when the molecule of interest occupies more than one site at a time. We’ll leave these important considerations for another day. Typically, the total sites can be measured. Resin vendors use terms such as “binding capacity” or “dynamic binding capacity” to advertise the capacity of their resins. The capacity is often dependent on the chemical of interest. The resulting relationship between c and q is no longer linear, it is represented by this equation:

When c is small, the denominator of this equation becomes 1, and the equilibrium equation looks like the linear equilibrium equation. When c is large, the denominator becomes K_eqc, and the resin concentration of the chemical is equal to the resin capacity, S_tot. When c is in between small and large, the isotherm bends over in a convex shape. This is shown in the graph below.

There are three basic conditions in preparative and industrial chromatographic operations. In the first, K_eq is very low, and there is little or no binding of the chemical to the resin. This is represented by the red squares in the graph above. This is the case with “flow through” fractions in chromatography, and would generally be the case when the chemical has the same charge as the resin. In the third, K_eq is very high, and the chemical is bound quantitatively to the resin, even at low concentrations. This is represented by the green triangles in the graph above. This is the case with chemicals that are typically only released when the column is “stripped” or “regenerated”. In these cases, the solution phase conditions are changed to turn K_eq from a large number to a small number during the regeneration by using a high salt concentration or an extreme pH. The second case is the most interesting, and is the condition for most “product” fractions, where a separation is being made. That is, when the solution phase conditions are tuned so that the desired product is differentially adsorbing and desorbing, allowing other chemicals with slightly higher or lower affinities to elute either before or after the desired product, it is almost always the case that the equilibrium constant is not such that binding is quantitative or non-existent. In these cases, the non-linearity of the isotherm has consequences for the shape of the elution peak. We will discuss these consequences in a future blog.

In a “Quality-by-Design” world, these non-linearities would be understood and accounted for the in the design of the chromatography operation. An excellent example of the resulting non-linearity of the results was shown by Oliver Kaltenbrunner in 2008. 

Relying on linear statistics to uncover this basic thermodynamic behavior is a fool’s errand. However, using basic lab techniques (a balance and a spectrophotometer) the isotherm for your product of interest can be determined directly, and the chromatographic behavior understood. This is the path to process understanding!

Epizootic hemorrhagic disease virus: a future troublemaker?

by Dr. Ray Nims

Epizootic hemorrhagic disease virus (EHDV) is a double-stranded RNA virus of family Reoviridae, genus Orbivirus. This is a non-enveloped virus of approximately 60-80 nm size. This arbovirus is transmitted by a biting midge of genus Culicoides, and is closely related to another Orbivirus, the bluetongue virus. Two serotypes are endemic to cattle in North America (EHDV-1 and EHDV-2); the infections caused tend to be subclinical (asymptomatic) and therefore may go undetected.

Infections in cattle are more prevalent in areas of widespread infection within the local deer population. As shown in the figure below, the geographic distribution of infection of deer populations with EHDV and bluetongue virus includes areas within the high plains and mountain states in which bovine serum production is high (Utah, Kansas, etc.).

From Daniel Mead, Risk of Introduction of New Vector-borne Zoonoses

There have been recent outbreaks of epizootic hemorrhagic disease in cattle in Indiana (2006) as well as other states; in Israel (2006); and in Turkey (2007).

Basis of Concern: EHDV has been isolated previously from a biologics manufacturing process employing a Chinese hamster ovary (CHO) cell substrate (Rabenau et al. Contamination of genetically engineered CHO-cells by epizootic haemorrhagic disease virus (EHDV). Biologicals 21, 207-214, 1993). The infection was presumed, but not proven, to originate from use of a contaminated bovine serum in the manufacturing process.

Regulatory Expectations. EHDV is not mentioned specifically in 9CFR 113.47 (Detection of extraneous viruses by the fluorescent antibody technique as a virus of concern for raw materials of bovine origin), although this regulation requires testing for the closely related bluetongue virus. EHDV would be expected to cause cytopathic effects in Vero cells, one of the detector cells used in the 9CFR 113.47 assay, therefore this assay should detect the virus in grossly contaminated bovine sera.

Mitigating Risk. Elimination of animal-derived materials (esp. bovine sera) from the manufacturing process should reduce the risk of experiencing this virus. If this is not possible, treatment of the sera should be considered. Gamma-irradiation of the frozen serum at the dosages normally used should be effective, judging from results obtained with REO virus, another member of the family Reoviridae (Gauvin, 2009).

Conclusions. EHDV has been found previously to contaminate a biologics manufacturing process employing a CHO-cell substrate. It is therefore a virus of concern for the biopharmaceutical industry. Risk of infection of biological products with EHDV through use of bovine-derived materials such as bovine sera may increase in the event of future outbreaks of this disease in cattle from serum-producing regions of North America or Australia. Risk may be mitigated through implementation of gamma-irradiation of bovine sera and of viral purification processes capable of removing and inactivating non-enveloped viruses such as MMV and REO.

FDA regulation of “combination” products

by Dr. Ray Nims

The FDA’s Office of Combination Products (OCP) was established in 2002 to shepherd combination products, those comprised of a combination of drug, biological, and/or device, through the review and regulation process. An example of a combination product is the drug-releasing stent (see figure below). The OCP does not conduct the reviews, but is responsible for: assigning the product to the appropriate FDA center; coordinating reviews involving more than one center; and working with agency centers to develop guidance and regulations to make the regulation of combination products “as clear, consistent and predictable as possible”. 

An example of a drug-releasing stent.

From Wikipedia

The complexity of combination products arises because the efficacy and safety of the individual constituent components (i.e., the drug, biologic, or device) must be considered alone, as well as within the context of the combination product. Because of this complexity, there is no single development paradigm for all combination products. The guidance recommends that combination product developers consider any prior approval/clearance of the constituent parts, as well as how their testing may be influenced by the interaction of the components. Factors that should be considered (from the guidance) include:

• Are the constituent parts already approved for an indication?
• Is the indication for a given constituent part similar to that proposed for the combination product?
• Does the combination product broaden the indication or intended target population beyond that of the approved constituent part?
• Does the combination product expose the patient to a new route of administration or a new local or systemic exposure profile for an existing indication?
• Is the drug formulation different than that used in the already approved drug?
• Does the device design need to be modified for the new use?
• Is the device constituent used in an area of the body that is different than its existing approval?
• Are the device and drug constituents chemically, physically, or otherwise combined into a single entity?
• Does the device function as a delivery system, a method to prepare a final dosage form, and/or does it provide active therapeutic benefit?
• Is there any other change in design or formulation that may affect the safety/effectiveness of any existing constituent part or the combination product as a whole?
• Is a marketed device being proposed for use with a drug constituent that is a new molecular entity?
• Is a marketed drug being proposed for use with a complex new device?

As with individual constituent drugs, biologics, and devices, the FDA will require that the combination products be manufactured according to current good manufacturing practices. In most cases, a single investigational application (IND or IDE) is submitted to enable the clinical trials planned for the combination product. The science and technology associated with the combination products should drive the selection of statistical approaches, sample sizes, study endpoints, and methods for active principle measurement and for evaluating possible interactions between components. It may be best to involve the FDA in these decisions.

Complexity for the regulation of combination products also stems from the fact that separate manufacturing processes may exist for the various constituent parts. Potential changes in any of the component manufacturing processes, subsequent to initiating clinical trials or post-market, will need to be evaluated for possible effects on the safety and efficacy of the combination product.

Combination products represent therapeutic modalities with great promise for advancing health care. We expect to see more and more pharmaceutical activity in this area going forward.

Is There Ever a Good Time for Filter Validation?

By Dr. Scott Rudge

What is the right time to perform bacterial retention testing on a sterile filter for an aseptic process for Drug Product? I usually recommend that this be done prior to manufacturing sterile product. After all, providing for the sterility of the dosage form for an injectable drug is first and foremost the purpose of drug product manufacturing.

But there are some uncomfortable truths concerning this recommendation

1. Bacterial retention studies require large samples, liters

2. Formulations change between first in human and commercial manufacturing, requiring revalidation of bacterial retention

3. The chances of a formulation change causing bacteria to cross an otherwise integral membrane are primarily theoretical, the “risk” would appear to be low

On the other hand

1. The most frequent sterile drug product inspection citation in 2008 by the FDA was “211.113(b) Inadequate validation of sterile manufacturing” (source: presentation by Tara Gooel of the FDA, available on the ISPE website to members)

2. The FDA identifies aseptic processing as the “top priority for risk based approach” due to the proximal risk to patients

3. The FDA continues to identify smaller and smaller organisms that might pass through a filter

Is the issue serious? I think so, risk of infection to patients is one of the few direct consequences that pharmaceutical manufacturers can directly link between manufacturing practice and patient safety, which is one of the goals of Quality by Design. Is the safety threat from changes to filter properties and microbe size in the presence of slightly different formulations substantial? I don’t think so, especially not in proportion to the cost to demonstrate this specifically. But the data aren’t available to demonstrate this hypothesis, because the industry has no shared database to demonstrate a range of aqueous based protein solutions have no effect on bacterial retention. There is really nothing proprietary about this data, and the only organizations that benefit from keeping it confidential are the testing labs. Sharing this data should benefit all of us. An organization like PDA or ISPE should have an interest in polling this data and then making a case to the FDA and EMEA that the vast majority of protein formulations have been bracketed by testing that already exists, and that the revalidation of bacterial retention on filters following formulation changes is mostly superfluous.

In the meantime, if you don’t have enough product to perform bacterial retention studies, at least check the excipients, as in a placebo or diluents buffer. A filter failure is far more likely due to the excipients than the active ingredient, which is typically present in much smaller amounts (by weight and molarity). By doing this, you are both protecting your patients in early clinical testing, and reducing your risk with regulators.

Oops, adventitious viral DNA fragments in a vaccine

by Dr. Ray Nims

On March 22, 2010, a press release from GlaxoSmithKline (GSK) announced that DNA from porcine circovirus had been detected in their rotavirus vaccine. According to GSK, the DNA ”was first detected following work done by a research team in the US using a novel technique for looking for viruses and then confirmed by additional tests conducted by GlaxoSmithKline”. As a result of this finding, the FDA “is recommending that US clinicians and public health professionals temporarily suspend the use of Rotarix as a precautionary measure. The FDA have also stated that they intend to convene an advisory committee, within approximately four to six weeks, to review the available data and make recommendations on rotavirus vaccines licensed in the USA. The FDA will also seek input on the use of new techniques for identifying viruses in vaccines.” The EMEA, on the other hand, does not appear to consider this finding to be a safety concern, citing the fact that porcine circovirus is not infectious for human cells and does not cause disease in humans.

Porcine circovirus

Source: Meat and Livestock Commission, UK

What is porcine circovirus? Porcine circovirus (PCV) is a member of the family Circoviridae, among the smallest of the known animal DNA viruses. It is approximately 17 nm in diameter, non-enveloped, with icosahedral symmetry. The virus was originally identified as a noncytopathic contaminant of the PK-15 porcine kidney cell line (Tischer et al., Zentralblatt Bakt Hyg A 226:153-167, 1974). Like many very small, non-enveloped viruses, PCV represents a challenge for removal and inactivation.

Why is this finding coming to light now, or stated another way, why wasn’t the PCV DNA detected when the vaccine was initially tested and approved for human use? The press release wasn’t specific as to method used. It has subsequently been revealed, however,  that these fragments were detected using sequence-independent amplification (deep sequencing; or as Eric Delwart calls it, metagenomics). The resulting library of amplified sequences is characterized by BLAST searching using identification algorithms. Confirmation in this case was obtained using microarray and PCR analyses. The sequencing techniques have been available for some time, and have proven useful for identification of viruses in an academic setting, though they have not been applied to safety testing of biopharmaceuticals until fairly recently due to the relatively high costs associated with the analyses.

The finding of viral DNA should not be equated with detecting the infectious virus in the product. The sequence-independent amplification, microarray, and virus-specific PCR assays used can detect viral nucleic acid, but as normally performed do not indicate whether infectious virus is present. Generally, with the possible exception of transforming viruses, it is the infectious virus that is of concern, not its DNA. Efforts to assess the presence of intact viral genomic material and of infectious porcine circovirus in this vaccine are most likely underway at this time.

The presence of the PCV viral sequences has provisionally been attributed to the use of porcine trypsin during the culture of the Vero cell substrate in which the vaccine is manufactured. The trypsin used had been gamma-irradiated to inactivate adventitious viruses prior to use. While it would be comforting to believe that the PCV DNA may simply have reflected carryover of non-infectious, lethally-irradiated PCV1 from the trypsin, the fact is that gamma-irradiation is not very effective at inactivating this small, non-enveloped virus (Plavsic and Bolin. Resistence of porcine circovirus to gamma irradiation. Biopharm Int. 14:32-36, 2001). In the case of porcine circovirus, there is little evidence to indicate that the virus is infectious or pathogenic for humans. So regardless of the outcome of the various ongoing studies, it is likely that the use of the GSK rotavirus vaccine will be re-instated after the FDA convenes and discusses the implications of this finding.

I predict that there will be more and more of this kind of revelation in the future as the sequencing techniques display stretches of viral or other contaminant DNA within samples of biopharmaceuticals. I would hate to see revelations like this impede the use of the sequencing technologies going forward, as these technologies are going to be very useful to the industry as rapid detection methods for contaminant identification.