Apoptosis induction by trastuzumab: Possible role of the core biopsy intervention

Identification of the mechanism of action of trastuzumab is an important issue since it should provide the basis for optimization of one of the first biomolecular treatments in clinical oncology. In their recent article published in the Journal of Clinical Oncology,1 Mohsin et al reported that short-term treatment with trastuzumab administered as a single agent in the neoadjuvant setting induces early tumor regression and is followed by a statistically significant increase of tumor cells apoptosis index as opposed to other biomarkers of cell cycle, survival, proliferation, and epidermal growth factor receptor expression. On this basis, they infer that apoptosis is the principal mechanism of the observed therapeutic activity and suggest that trastuzumab may work by cytocidal rather than cytostatic mechanisms. Since we have also conducted a smaller pilot study on the mechanism of action of preoperative trastuzumab in patients with primary operable breast tumors overexpressing HER2,2 we would like to convey attention to the following points. First, the failure to identify any down-modulation of the HER2 receptor, as well as any blockage of its signaling pathway by Mohsin et al,1 is fully consistent with the data reported last year as the results of our pilot study.2 Second, although patients included in our study had smaller (cT1-3, N0–2) and operable tumors, we also found that 4 weeks of treatment with trastuzumab alone were sufficient to obtain one complete remission and four partial responses (response evaluation criteria in solid tumors criteria) out of 11 treated patients, thus confirming that short-term trastuzumab monotherapy may be clinically effective by inducing rapid responses most likely related to cytocidal rather than cytostatic effect. However, we suggest caution about the interpretation that such cytocidal effect induced by trastuzumab may be simply directed through early apoptosis. Indeed, the apoptotic cells increase observed by Mohsin et al was quite consistent in biopsies performed either at day 1 or at day 8 (grouped as within 1 week) from the first injection of trastuzumab, but apoptotic cell number was found to return to baseline values at later time points. The authors comment that most chemotherapeutic agents also induce apoptosis within 24 hours of exposure, but it is well known that these drugs display a peak of concentration at time of injection, which quickly decreases, and their activity is therefore very rapid. By contrast, the steady state of trastuzumab is reached only after three to four injections, and an early peak of its activity is quite unlikely. In our opinion, it must be considered even hypothesisized—that such an early apoptosis may not be directly linked to the monoclonal antibody therapy, but rather to the core biopsy maneuver. Indeed, while addressing the same issue of trastuzumab activity, we found initially in our pilot study that infiltration by lymphocytes was less than 50% in core biopsies before treatment versus 90% of the tumors removed after four administrations of trastuzumab intravenous, thus suggesting an active role of trastuzumab in recruiting immune cells.1 But further investigation of tumors from untreated patients, revealed a similarly significant increase of infiltrating cells in the surgical specimens compared to biopsies since the frequency of cases with a strong infiltrate passed from 30% (3 of 10) in core biopsies to 90% (9 of 10) at surgery (P _ .01), thus indicating that the core biopsy intervention by itself induces inflammation and recruitment of lymphoid cells at the tumor site. Inflammation is inevitably accompanied by a cytokine storm that induces apoptosis of the surrounding cells.3,4 Therefore, tumors from untreated or placebotreated patients should be analyzed to discrimate between the roles played by trastuzumab and by core biopsyinduced inflammation. Lastly, even when considering apoptosis responsible at least in part for the cytocidal effect, it remains to be seen whether it is induced through a direct or indirect Fc_ receptors-dependent mechanism(s). Currently, we favor this latter hypothesis since we found a positive correlation between tumor reduction and antibody dependent cell cytotoxicity that leads to killing of target tumor cells not only by perforin/granzyme lysis, but also through TNF/FasL apoptosis.5 Mohsin et al have focused all their attention on tumor characterization without taking into account the role played by patients’ immune system, therefore their conclusion that “apoptosis provides a molecular explanation for both trastuzumab therapeutic efficacy and its combination with cytotoxic chemotherapy” does not seem sufficiently substantiated at present