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PFIZER DIABINESE BIOEQUIVALENCE TO GENERIC CHLORPROPAMIDE

Executive Summary

PFIZER DIABINESE BIOEQUIVALENCE TO GENERIC CHLORPROPAMIDE will be assessed in a cross-over study comparing the Pfizer brand to two generic versions of chlorpropamide at the University of Washington in Seattle. In a July 1 letter to FDA, Pfizer said that the company is sponsoring a study in diabetic patients controlled on Diabinese "in an attempt to assess the validity of our concerns [over generic chlorpropamide products] in the clinical situation." The firm said data from the study is expected to be analyzed "by early 1986." The study, involving 30 patients, began in late June-early July. Pfizer's letter was written in response to two letters FDA sent the firm in April objecting to promotional material comparing Diabinese with generic versions of the drug. In April 3 correspondence, the agency commented on a February letter Pfizer sent to state formulary cmte. members. FDA stated that Pfizer's letter "contains representation and other claims which may misleadingly suggest that prescribing approved generic chlorpropamide for patients taking Diabinese will result in clinical failures." The agency asserted: "We regard this entire letter as without basis and an attempt to falsely malign both the generic drug industry and, more importantly, FDA's ANDA system for ensuring therapeutic equivalence in approved generic products. It can only be viewed as an attempt to falsely confuse all healthcare practitioners about approved generic chlorpropamide products." Pfizer's letter to the formulary cmte. members alleged differences in dissolution characteristics, bioequivalency standards and composition betweeen Diabinese and generic chlorpropamide products. For instance, the letter states that "Pfizer's standard for dissolution release by current USP test requires that Diabinese tablets exhibit a greater amount of drug substance dissolved in a significantly shorter period of time than does the USP standard" (USP standard 75% in 60 minutes versus Pfizer's 80% in 15 minutes). The firm adds that in order for ANDA approval, a drug must meet only USP standards; "thus there is the potential for significant differences in rates of dissolution as well as other quality parameters between Diabinese and the generic chlorpropamides." FDA replied that while Pfizer says this is a significant difference, "you fail to mention that this is also clinically insignificant and that any product meeting the USP standard (including bio-requirements), the minimum requirement for all ANDA approvals, and found therapeutically equivalent to Diabinese, will not cause a 'treatment' failure." Similarly, FDA questioned Pfizer's implication that differences in bioequivalency standards or compendial grade excipients would make a significant difference between chlorpropamide products. In a second letter to Pfizer, dated April 15, FDA questioned a write-in campaign in lllinois where physicians requested that the state's Dept. of Public Health keep Diabinese on the state formulary rather than replacing it with a generic version as had been proposed. "It has been suggested that an organized effort has been instrumental in coordinating and soliciting 'independent' comments from practitioners to not admit approved generic chlorpropamide tablets to the formulary," FDA stated. "While this activity on the surface may not seem to be in conflict with FDA's statute, regulations, etc., it is sometimes difficult to determine the full story to such questions as what oral or written information may have been used to persuade practitioners to 'petition' their local regulators," the agency said. FDA noted that it has received copies of the letters protesting the formulary change and that they "are virtually identical and even appear to be prepared by the same word processing facility." The agency asserted: "We are concerned that the apparent 'chance' issuance of your Feb. 14, 1985 'Dear Doctor' letter providing a chlorpropamide drug experience report form, a February 1985 'Dear Formulary Cmte.' letter followed by 'grassroots' write-ins (petitions) by physicians suggest more than random chance. Our interest over these apparent 'independent' events that appear to discredit approved therapeutically equivalent chlorpropamide products is that they all make strong negative connotations about FDA's ANDA approval process and offical USP/NF requirements." FDA's correspondence with Pfizer is the second time the agency has objected to a company's anti-generic promotional material. In November 1984, FDA criticized a Sandoz advertisement of Mellaril (thioridazine), which FDA said implied there were significant differences between thioridazines.

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