Glen Bolton

Human plasma provides a rich source of therapeutic medicines, including gamma globulins, coagulation factors, albumin, alpha anti-trypsin and others. In 2001, sales of immuno gamma-globulin (IgG) were estimated at $2 billion with a production rate of 50 metric tons for the year.

  A number of therapeutic products have been introduced including Gammimune from Bayer, RhoPhylac from ZLB Behring and Octagam from Octapharma.  

Hyperimmunes (IgGs with high levels of antibodies targeted at a specific antigen such as a virus) are also under development for biodefence applications such as anthrax or smallpox. These are useful as either therapeutics or preventative measures. 

In the past, contamination of source plasma by enveloped viruses (HIV, HCV) has led to the contamination of plasma products and patient infection.

  In response, the industry adopted a multilayered virus safety strategy that includes source plasma control (donor screening, plasma testing), product manufacturing (plasma pooling, clearance steps in plasma product manufacturing, current good manufacturing practice [cGMP]), product testing and pharmacovigilence [patient monitoring, recalls]). The enveloped virus problem has been addressed by the adoption of virus clearance technologies such as solvent detergent, heat, low pH, caprylate, chromatography and large virus filtration. Regulatory authorities have issued regulations describing requirements for marketing approval.

Plasma product safety concerns are focused on the smaller nonenveloped viruses (HAV, B19) and new agents (TSE, WNV, SARS). These small, nonenveloped viruses can be more difficult to inactivate or remove.

  The solvent detergent method, which is effective in preventing enveloped virus replication (through dissolution of their lipid envelope coating), is ineffective at inactivating nonenveloped viruses. The severe treatment conditions required for some nonenveloped virus inactivation (for example, 100 °C for PPV) can lead to significant product losses and potential formation of neo-antigens. 

Virus filtration is a useful clearance method for nonenveloped viruses.

  Filters are inert and do not degrade the product or add any foreign substances requiring removal. They are also capable of providing high product yield, can be validated using scaledown models and in-process integrity tests, and can be conveniently integrated into manufacturing processes. 

Virus filtration is a well-established unit operation in biopharmaceutical processes and has been implemented in a number of approved products dating back to the mid 1990s. Mononine, a Factor IX product developed by Armour Pharmaceutical Company, was approved in 1992 using an ultrafiltration process for viral reduction.

The Millipore Viresolve NFP is a normal-flow parvovirus-removal filter with a patented membrane structure for strength and high flow. The NFP filter contains three layers of a highly asymmetric membrane as shown in Figure 1. The filter retains 4 ≥ logarithmic reduction value (LRV) of 20 nm diameter parvoviridae with high flows (75–150 L/m

 /h at differential pressure of 30 psi [2.1 bar]) and high capacities (100–200 L per 0.5 m

  area filter). Features of the filter are listed in Table 1.  

The development of a parvovirus filter process requires examining how the design variables (for example, feedstock and operating conditions) affect the performance parameters (for example, sizing, product recovery and LRV). Filter sizing is determined by capacity and flux. Capacity can be conveniently characterized by the 

 model, based on the gradual plugging of filter pores.

  When the volume of a batch is fixed at 

 and the allowable time to process a batch is fixed at 

, the required area will be described by Equation 1.  

When the stream is relatively nonfouling and the batch time is low, the size will primarily be determined by the initial flux, 

. With longer batch times and dirtier streams, 

 determines the size. The concept is easier to grasp by using the sizing index a, a dimensionless term shown in Equation 2 that indicates whether membrane area requirements will primarily be determined by membrane flux or membrane capacity. 

The sizing equation can be simplified to Equation 3 when using the sizing index. 

The sizing index provides a simple indication of the factors that determine membrane area requirements for a process. When α is greater than 10, the process is flux-limited — more than 90% of the area required is determined by the initial flux. When the value of α is less than 0.1, the process is capacity-limited — more than 90% of the area required is determined by the capacity. 

Table 1 Features of Viresolve NFP filter.			

As shown in Figure 2, filters with low α (Filter A) outperform filters with high α (Filter B) at short process times. This allows NFP filters to offer manufacturing flexibility, such as operating in series with other process steps. Most small virus filters on the market have low flux and high capacity and, therefore, have high α values. These filters are only economical at extended process times. 

Figure 1 Magnified image of Viresolve NFP membrane.			

Increasing pressure can improve filtration throughput, which is why 50 psig (3.5 bar) is usually the optimum operating point. Representative data showing this effect is shown in Figure 3, where using an average flux incorporates the effects of both a high initial flux and a high capacity. NFP devices are rated to 80 psig; most customers prefer not to run above 50. 

Protein concentration in the feedstock impacts capacity. Test data in Figure 4 show the effect. This relationship can be modelled as shown in Equation 4. 

 is subjected to various levels of dilution to improve the 

 the filtration sizing can be determined in terms of the process concentration. Equation 5 is a mass balance. Plugging this into Equation 3 allows the size to be determined as in Equation 6. 

Articles

Human plasma provides a rich source of therapeutic medicines, including gamma globulins, coagulation factors, albumin, alpha anti-trypsin and others. In 2001, sales of immuno gamma-globulin (IgG) were estimated at $2 billion with a production rate of 50 metric tons for the year.1 A number of therapeutic products have been introduced including Gammimune from Bayer, RhoPhylac from ZLB Behring and Octagam from Octapharma.