CHAPTER 9

Standardized Extracts

In attempting to find a more consistent and accurate way to measure the strength of allergy extracts, the Food and Drug Administration (FDA) has, in its quest for standardization, created something of a dilemma. One definition of a dilemma is “a situation that requires one to choose between two equally balanced alternatives.” In the case of a change from nonstandardized to standardized extracts, however, the alternate definition seems more applicable: “a predicament that seemingly defies a satisfactory solution.”

THE NEED FOR ANTIGEN QUANTIFICATION

It has long been recognized by allergists that allergenic extracts are quite different from other forms of medication. It is unfortunate that the lay public, most third-party payers, and many physicians not involved in allergy testing and immunotherapy do not seem to understand the reasons why allergenic extracts do not follow the familiar pattern established for commercially prepared drugs. It is nonetheless necessary to understand the unique nature of allergenic extracts to carry out allergy diagnosis appropriately and administer immunotherapy properly. One special area of confusion during the last decade has been the introduction of new formulations of allergy extracts that conform to the requirements of the FDA for “standardization.”

In general, drugs from all manufacturers may be assumed to be uniform in potency, conforming to the standards set by such agencies as the FDA and the United States Pharmacopeia (USP). These drugs consist of a designated amount of clearly identified, chemically defined medication at a measured level of potency, contained in an inert vehicle. The same amount of chlorpheniramine is expected to be contained in and released from a 4-mg tablet from any manufacturer or any drug lot. Allergenic extracts, on the other hand, are mixtures derived from active biologic material by a variety of means. The extract generally contains not only the desired allergen, but also several other organic substances present in varying amounts in the material from which the extract is made (Table 9-1). The amount and potency of immunologically active material varies from one batch of extract to another, even when identical extraction methods are used, because of natural variations in the level of allergen in the particular batch of pollen or mold from which the extract is made. To minimize this, most antigen manufacturers utilize pollen gathered during more than one year in making a given batch of antigen. This helps, but does not totally resolve the problem. To date, it has not been possible to prepare allergenic substances synthetically on a commercially feasible scale. The allergenic substances are grown by nature, and nature varies in uniformity. Extracts of the same allergen, therefore, are not exactiy uniform, varying in biologic activity between manufacturers, or even in lots from the same manufacturer, by up to a thousandfold at times.

This discrepancy among various samples of a product has been recognized since the earliest days of immunotherapy. It has led to a constant search by practitioners and researchers alike for a means of accurately providing quantification, and thereby some form of standardization, for allergy extracts. This goal has never been fully achieved. Today, medicine is closer than ever before to achieving a degree of standardization for allergenic extracts, but perfect standardization for all antigens has not yet been reached. Despite having weathered the transition period between the use of extracts quantified by historically proven methods and of the govern mentally mandated standardized antigens, confusion continues to exist in this area.

To understand the new methods of defining extract potency, it is necessary to know how antigens have been previously and are currently quantified. In this regard, a brief historical review of the various designations of allergenic extracts may be of value.

Leonard Noon is considered by many to be the father of allergy testing and immunotherapy. As early as 1911, he felt it necessary to establish some form of standardization for allergy testing to compare the degree of sensitivity to different allergens manifested by various patients. His approach resulted in the designation of what is now known as the Noon unit, which he defined as the amount of antigenic material that can be extracted from 0.001 mg of Phleum (timothy grass) pollen. When this amount of antigen was placed in the conjunctival sac of an allergic patient, the reaction could be compared on a unit basis with the amount of extract required to produce a similar reaction in another patient. Although the principle was valid for comparison purposes between patients, the procedure never become popular as a means of standardization. Nevertheless, today one may find references to a Noon unit, representing 1 μg of antigenic material.

Another historical designation for allergen standardization is the protein nitrogen unit (PNU). This standard was introduced in 1933 and is still in use in some situations. However, it has been well demonstrated that the PNU does not indicate the allergenic potency of the extract, but rather tends to serve as a measure of the amount of all proteins present (both allergenic and nonallergenic).² It is also worthy of note that there is no reliable means of converting between strength measured in weight/volume (w/v) (see below) and PNUs.

The most common approach to allergen standardization, and the one still in the widest use today, is the w/v formulation. This is simply the amount of dry allergen in grams extracted in a given amount in milliliters of diluent. As an example, 1 g of pollen extracted in 20 mL of diluent would represent a 1:20 w/v concentration. This designation is simple to understand. It does not always represent biologic potency in a reliable manner, but it has served for decades as a practical means of designating allergen extract strengths for testing and immunotherapy. Those practitioners familiar with the weaknesses of the system, though realizing that equality in w/v designation does not guarantee bioequivalence, have simply allowed an appropriate margin of safety or relied on titration and vial tests when changing to a new stock extract or to a new supplier. Even though standardized extracts are becoming more common, the strength of the majority of allergenic extracts is designated by specifying the w/v value (Fig. 9-1).

A variety of methods have been developed in recent years in an attempt to improve both qualitative and quantitative uniformity of extracts,³ the most important of which is quantitation using the standardized allergy unit (AU) or bioequivalent allergy unit (BAU). Over the past few years, the FDA has continued its efforts to see that all allergenic extracts available eventually are standardized using this method. Standards are developed by the International Union of Immunologic Societies (IUIS), in cooperation with the World Health Organization. When standard reference extracts are accepted by the FDA, they are made available to antigen manufacturers. Antigenic extracts are then developed, which are compared with these standards through both in vivo and in vitro reference methods such as radioallergosorbent test (RAST) inhibition and dilutional titration. Antigens thus produced are then labeled as containing BAUs or AUs in varying concentrations. Standards are available from the IUIS in vials labeled as containing 100,000 international units per mL. It should be noted, however, that extracts developed and tested against these standards may be of varying strengths (Table 9-2). The FDA may also make standards available that have been chosen from commercially available extracts. Standardized extracts currently exist for several antigens (Table 9-3). This standardization is accomplished in several ways.