Early Acquired Resistance to Endocrine Therapy: Extending the Neoadjuvant Model.

Abstract

Abstract Background: Neoadjuvant endocrine therapy allows biological studies of de novo response and resistance but not of acquired resistance based on clinical endpoints as surgery is usually carried out after around 4 months. Response is associated with a significant fall in Ki67 (proliferation) after 2 weeks treatment. However, at 12/16 weeks, c.15% of tumours show recovery of Ki67. We hypothesized that this recovery reflected rapidly acquired treatment resistance and have assessed its clinical relevance and association with tumour biomarkers.Methods: Ki67, ER, PgR and HER-2 (with FISH for 2+ cases) IHC measurements taken at baseline, 2 weeks and 12/16 weeks during neoadjuvant aromatase inhibitor (AI) treatment were available from three previously reported clinical trials [1-3] for a total of 177 ER+ patients. To allow pooling of data, ER and PgR levels were expressed as Allred scores. Long-term clinical follow-up data were available for 142 patients who received endocrine therapy within the IMPACT trial. Categories of response/resistance based on Ki67 change were created and patients classified accordingly: de novo resistance (<50% decrease in Ki67 from baseline to 2 weeks), acquired resistance (>50% decrease in Ki67 from baseline to 2 weeks, but overall fall at 12/16 weeks <50%) and maintained response (>50% decrease in Ki67 from baseline to 2 weeks and overall fall >50%).Results: Patients defined as showing acquired resistance had significantly worse RFS than those defined as showing maintained response (HR: 3.73, 95%CI: 1.18-11.79, p=0.025) and very similar to those with de novo resistance (HR: 3.66, 95% CI: 1.41-9.50, p=0.008). More HER-2+ cases showed Ki67 recovery than would be expected by chance (acquired resistance: 5/19 HER-2+, maintained response: 5/100 HER-2+; p=0.02). There was a weak suggestion that pre-treatment ER level may be lower for patients showing Ki67 recovery than those with persistent response (p=0.11), but no evidence was seen for an effect of pre-treatment PgR level (p=0.52).Conclusions: Tumours that exhibit an early Ki67 fall followed by Ki67 recovery at 12/16 weeks are associated with poorer RFS than those showing maintained suppression of Ki67 suggesting that Ki67 may act as a valid intermediate marker of acquired resistance. HER-2 positivity is associated with development of early acquired Ki67-based resistance, but other mechanisms for early acquired resistance exist.Supported by The Mary-Jean Mitchell Green Foundation.[1] Dowsett M et al., J Clin Oncol 2005. 23(11): 2477-92[2] Smith IE et al., J Clin Oncol 2007. 25(25): 3816-22[3] Miller WR et al., J Clin Oncol 2009. 27(9): 1382-7 Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2005.