Which generic anastrozole formulation has inactive ingredients that differ from the brand Arimidex (anastrozole)?

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Inactive Ingredient Differences Between Arimidex and Generic Anastrozole

The primary difference in inactive ingredients between brand Arimidex and generic anastrozole formulations is that Arimidex contains "polyethylene glycol" and "lactose" (non-monohydrate form), while generic anastrozole contains "macrogol" (which is polyethylene glycol but listed differently) and "lactose monohydrate" (a specific hydrated form of lactose). 1, 2

Detailed Formulation Comparison

Brand Arimidex (Reference) Inactive Ingredients:

  • Lactose (unspecified form)
  • Magnesium stearate
  • Hypromellose
  • Polyethylene glycol
  • Povidone
  • Sodium starch glycolate
  • Titanium dioxide

Generic Anastrozole Inactive Ingredients:

  • Lactose monohydrate (specified hydrated form)
  • Magnesium stearate
  • Hypromellose
  • Macrogol (alternative name for polyethylene glycol)
  • Povidone
  • Sodium starch glycolate
  • Titanium dioxide

Clinical Significance

These minor differences in inactive ingredient specifications do not affect bioequivalence or clinical efficacy. The generic formulation has been demonstrated to be bioequivalent to Arimidex, with 90% confidence intervals for Cmax, AUC0-t, and AUC0-∞ all falling within the 80%-125% acceptance range required by regulatory authorities 3.

Key Points:

  • The active pharmaceutical ingredient (anastrozole 1 mg) is identical in both formulations
  • Both formulations contain the same inactive ingredients, just with slightly different nomenclature or specifications (lactose vs. lactose monohydrate; polyethylene glycol vs. macrogol)
  • Macrogol is simply another name for polyethylene glycol - they are the same substance
  • The lactose monohydrate specification in the generic indicates a specific crystalline form with one water molecule, versus unspecified lactose in the brand

Clinical Implications:

  • Patients can be switched between brand and generic formulations without concern for altered efficacy
  • The recommended dose remains 1 mg daily for both formulations 4
  • No dose adjustment is needed when switching between formulations
  • Both formulations have demonstrated equivalent estrogen suppression capabilities in postmenopausal women

For patients with lactose intolerance, both formulations contain lactose-based excipients and may cause similar issues, though the monohydrate form in generics is a more defined specification.

References

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