The key factors in considering specific delivery systems are safety, stability and efficacy. The success of the product development of biopharmaceutical therapeutic agents requires close interactions of interdisciplinary sciences encompassing molecular biology, fermentation, process development, protein chemistry, analytical biochemistry, pharmacology, toxicology, preformulations, formulations, clinical development, quality assurance, scale-up, bulk manufacturing, aseptic manufacturing, regulatory affairs, marketing and others (Bontempo, 1977)
Protein instability have been greatly covered by several investigators (Hageman, 1988; Wang and Hansen, 1988; Manning et al., 1989; Geigert, 1989; Privalov and Gill, 1988; Nozhaev et al., 1988; Pace et al., 1989; Timasheff et al., 1989; Arakawa at al., 1991; Schein, 1990; Glynn, 1980). Chemical reactions such as oxidation, deamidation, proteolysis, racemization, isomerization, disulfide exchange, photolysis, and others, will give rise to chemical instability.
It is critical that when this happens, the denaturation mechanisms must be identified in order to select appropriate stabilizing excipients. These chemical excipients may be in the form of amino acids, surfactants, polyhydric alcohols, antioxidants, phospholipids, chelating agents, and others.
a. Chelating Agents
During fermentation, purification of the bulk active protein residual metal ions could be present as it contacts stainless steel, iron or copper surfaces. The use of a chelating agent is a requirement and recommended dosages may range from 0.01 to 0.05% to bind or chelate the metal ions present in the solution.
Ascorbic acid, monothio-glycerol or alpha tocopherols have been used for this purpose. A recommended dose (Powell et al., 1998) would range from 0.05 to 0.1%.
A multidose formulation requires an antimicrobial agent, or in most references, it is referred to as preservatives. The effectiveness of the preservative must comply with USP requirements and meet the “Antimicrobial Effectiveness Test”. The most often used preservatives for protein or recombinant products are phenol at 0.3 to 0.5%, chlorobutanol, also at 0.3 to 0.5%, and benzyl alcohol at 1.0 to 3.0%.
d. Glass vial selection
The selection of a vial first of all should be of Type I glass as classified in the USP. All experimental work begin with this type of glass, hence, determining early enough the interactions of the proteins with the glass surfaces.
e. Rubber stopper selection
Equally important in screening initial preformulations is the selection of a compatible rubber stopper with protein solutions. The variety of rubber stoppers composition in parenteral formulations of biopharmaceuticals requires studies on compatibility with proteins, chemical extractants from the rubber composition into the protein solution over period of stability at varying temperatures.
f. Membrane Filter selections
All the protein formulations are aseptically filled for final sterilization of the product, the selection of the membrane filter and its specific media compositions is crucial. The chemical nature of the filter membrane and the pH of the protein adsorption (Hansen et al.,1992) are two of the most important parameters.
Of all the filters tested (unpublished data) polyvinylidene difluoride, polycarbonate and polysulfone were the most compatible with several proteins with minimal amounts of protein binding and deactivation.
The most demanding and exact technologies that a formulation group cannot be without is analytical technology. The development of quantitative methods of any active protein or peptide is of paramount importance toward the successful evaluation of chemical and physical stress.
These quantitative methods must eventually detect potential chemical degradants, contaminants, and impurities induced by oxidation, deamidation, proteolysis, and disulfide exchange. Physical instability such as aggregation, denaturation, adsorption, and precipitation must also be detected and quantitated.
The goal of the analytical methods must be reproducible and validatable to ensure regulatory compliance for the FDA, and must have confidence in the quality of a potentially successful marketable product. Some of the most often used analytical methods are the chromatographic techniques such as size exclusion, ion exchange, affinity chromatography, and reverse phase chromatography.
Others equally important methods are the electrophoretic techniques such as polyacrylamide gel electrophoresis and isoelectric focusing, western blots, combined electrophoresis and isoelectric focusing (two-dimensional electrophoresis). Bioactivity methods are another major area in assisting with advances for biotechnology product development. These are in vivo whole animal bioassay, cell culture bioassay, immunoassay, and biochemical assay.
Glass vials, and more recently plastic vials, are the representative containers which the product comes into contact with.
The screening and final selection should:
Filter media test most often tested for protein formulation with the lowest adsorption and maximum compatibility are mixed esters of cellulose acetate, cellulose nitrate, polysulfone and nylon 66. Membrane filters must be tested for compatibility with the active drug substance and selected for formulations if they have the lowest adsorption and maximum compatibility with the product.
Glass vials, elastomeric closures and filtration membrane extractables
All pharmaceuticals and materials for medical items are carefully screened and tested for extractables. Some pertinent degradants from glass are silica lamination, especially in phosphate buffer after six or more months on stability. Citrate and EDTA can induce complexing agents as well as high levels of sodium, aluminum, barium and iron.
Elastomeric closures can leach out accelerators such as mercaptobenzothiazole and tetramethyl thiuran disulfide; activators such as zinc oxide. Lubricants will excrete stearic acid as inert components and antioxidants will excrete hindered phenol.
Time Pressure Filling
A time pressure filling system in its simplest form, includes a product supply vessel under controlled pressure, a valve to open and close the product flow from the tank to the filling needle, and a clock or timing device to repeatedly control the amount of time that the valve is open.
In this system, the product supply pressure is controlled precisely and is, at the same time monitored continuously, so that a significant change in supply pressure automatically triggers a change in the valve open time for the current fills. This gives precise flow orifices to maintain a controlled relationship between product supply pressure and product flow through the filling needles and into vials.
This time pressure filling system is tied into a check weigh system that periodically weighs samples of filled vials and feeds the fill volume information back into the control system to make automatic fill adjustments. Weight values are tabulated on the human-machine-interface ((HMI) or control panel) so that the operator nay keep tract of the process throughout the fill campaign. Control and alarm limits for the specific campaign are preset, and fill volume values falling outside of alarm limits automatically trigger a vial reject function and may trigger the machine to revert to a dose-in until values come back into limits. During a typical dose-in mode, a single empty vial or select number of empty vials are released onto the vial transport system to pass under the filling station. The vials are pre-weighed (tare weigh) and then are re-weighed after filling (gross weigh) to determine the fill volume at each filling needle. An adjustment is made, either manually or automatically. This process is repeated until the fill volumes are all within control limits. Once the dose-in process is completed, the machine is returned to the normal filling mode, with a continuous flow of vials.
Another feature of this system is temperature compensation. After the system have “learned” the relationship between product flow through the orifice and temperature (a result of product viscosity change with temperature), an automatic adjustment of fill volume is made if a significant temperature change is measured. That is especially valuable after machine stoppage when the product may have warmed.
Fill by weight
The product supply tank may or may not be pressurized, and a flow orifice may be used to improve repeatability.
In a fill weight system, each vial rests on an electronic balance or load cell during the fill. A valve downstream of the product supply tank is opened to begin the fill. When the fill volume is near the target, the valve is closed, triggered through the control processor. A sophisticated system learns the actual dispensed volume during the valve closing, so that the closing of the valve may be anticipated based on the total weigh to that point.
A distinct advantages of the fill weight system is the 100% control and verification of fill volumes. A disadvantage is the low cycle rate, which may require multiple filling needles and load cells to reach an acceptable machine throughput rate and to reduce the risk of spillage that impacts the scales or the transport mechanism.
Open Active Restricted Access Barrier System (oRABS) and closed Restricted Access Barrier System (cRABS) can both be integrated with filling line machine, electronic sievers and millers for better containment of potent and hazardous products. The rigid walls and the panels provide physical separation during aseptic processing operations.
Mass Flow Filling
The product supply tank may be pressurized at a control pressure, or may use a precisely controlled liquid level (gravity feed). For small fills, common in pharmaceutical filling, a pressurized system provides more precise fill volume control and acceptable cycle rates.
Downstream of the product supply tank is a mass flow meter, which measures the mass flow of the liquid passing through it. A valve below the mass flow meter is opened to begin the fill. The mass flow meter continuously measures the flow, and triggers the closing of the valve when the target fill volume has been reached.
The mass flow filling system may use a flow orifice to more precisely control the flow rate.
A common mass flow meter type used for pharmaceutical fillings uses the Coriolis principle to measure mass flow. In a Coriolis meter, the liquid phases through one or more vibrating stainless steel tubes. Sensors at the inlet and outlet ends of the tubes measure the phase shift in the tubes’ oscillation. For pharmaceutical filling of injectable drugs, the inside of the tubes, the end connections, and the welds are held to the quantity standards of other product contact parts, such as product feed piping and filling needles.
A gravimetric filling system is simply a system that uses gravity, or a controlled liquid height, to provide a consistent pressure at the metering device near the filling needle. A simple time pressure filling system or a fill by weight system may be gravimetric. These system use a “clock” and a valve (time pressure) and a scale and a valve (fill by weight) for metering doses.
Peristaltic Pump Filling
The peristaltic pump uses a rotor in which multiple rollers are mounted. Partially surrounding the rotor is a stationary, curved shoe or “anvil”. A flexible, hollow tube (usually silicone rubber) is pinched between each roller and the shoe. As the rotor rotates, fluid inside the tube is driven forward as the pinched portion of the tubing advances.
The piston pump, along with the rolling diaphragm pump, may be compared to a syringe in that they both employ a moving piston inside a stationary cylinder to displace a precise amount of liquid. As the piston moves upward, liquid is forced out of the pump and when it moves downward, liquid is drawn into the pump.
Rolling Diaphragm Pump
The rolling diaphragm pump uses a flexible membrane (diaphragm) attached to the pump body at its outside diameter and to the piston at its inside diameter. A space between the piston and the body internal body cylinder allows the diaphragm to be “doubled” and to “roll” as the piston moves up and down. Vacuum is applied to a port in the lower portion of the pump body to maintain the shape of the diaphragm and to pull the piston downward on the refill portion of the filling cycle. Typically, the product supply has a slight overpressure.
The potency of new drugs continues to increase, particularly in the area of biopharmaceuticals and other biotech drugs. An increase in drug potency often results in a decrease in the volume required per dose. Therefore, equipment used for final filling is being used to fill decreasing volumes of liquid pharmaceuticals. It is not uncommon to dose liquid volumes below 1.0 mL using high speed filling systems. Another strong trend is the filling of parenteral liquid pharmaceuticals directly into final administration devices, such as prefilled syringes. Besides being convenient for caregivers and patients, these administration devices eliminate product waste attributed to the use of vials and separate syringes to prepare and deliver product doses. Transfer from the vial to the syringe always results in product being thrown away, which is increasingly a cost issue.
Combining the two trends leads to the requirement to fill decreasingly small volumes of drug products into final administration devices. Executing this scenario correctly requires the highest level of dosing accuracy and precision.