Novel adenoviral vector induces T-cell responses despite anti-adenoviral neutralizing antibodies in colorectal cancer patients

Abstract

First-generation, E1-deleted adenovirus subtype 5 (Ad5)-based vectors, although promising platforms for use as cancer vaccines, are impeded in activity by naturally occurring or induced Ad-specific neutralizing antibodies. Ad5-based vectors with deletions of the E1 and the E2b regions (Ad5 [E1-, E2b-]), the latter encoding the DNA polymerase and the pre-terminal protein, by virtue of diminished late phase viral protein expression, were hypothesized to avoid immunological clearance and induce more potent immune responses against the encoded tumor antigen transgene in Ad-immune hosts. Indeed, multiple homologous immunizations with Ad5 [E1-, E2b-]-CEA(6D), encoding the tumor antigen carcinoembryonic antigen (CEA), induced CEA-specific cell-mediated immune (CMI) responses with antitumor activity in mice despite the presence of preexisting or induced Ad5-neutralizing antibody. In the present phase I/II study, cohorts of patients with advanced colorectal cancer were immunized with escalating doses of Ad5 [E1-, E2b-]-CEA(6D). CEA-specific CMI responses were observed despite the presence of preexisting Ad5 immunity in a majority (61.3 %) of patients. Importantly, there was minimal toxicity, and overall patient survival (48 % at 12 months) was similar regardless of preexisting Ad5 neutralizing antibody titers. The results demonstrate that, in cancer patients, the novel Ad5 [E1-, E2b-] gene delivery platform generates significant CMI responses to the tumor antigen CEA in the setting of both naturally acquired and immunization-induced Ad5-specific immunity.

Trial registration: ClinicalTrials.gov NCT01147965.

Conflict of interest statement

Conflict of Interest

The following authors declare financial conflict of interest: Elizabeth S. Gabitzsch, Younong Xu, Stephanie Balcaitis, Rajesh Dua, Susan Nguyen, Joseph P. Balint, Jr., Frank R. Jones are employees of Etubics Corporation. All other authors do not have any conflict of interest.

Figures

Figure 1. CEA-directed CMI responses in treated patients

CMI (IFN-γ secretion) was assessed at baseline (Pre) and after administrations of Ad5 [E1-, E2b-]-CEA(6D) (Post). The highest CMI responses (regardless of time point) observed in the patients after treatment revealed a dose response. The highest CMI levels occurred in patients that received the highest dose of 5×1011 VP (Cohort 5). The CMI responses for Cohort 3/Phase II and Cohort 5 were significantly elevated (Mann-Whitney test) as compared to their baseline (Pre) values. Specificity of the responses was demonstrated by the lack of reactivity with the irrelevant antigens β-galactosidase and HIV-gag (data not shown). For positive controls, PBMCs were exposed to concanavalin A (data not shown). Horizontal line and error bar indicate the mean ± SEM for each Cohort.

Figure 2. Ad5 immune responses

Ad5 NAb titers (A) and CMI responses (B) to Ad5 were determined in patients at baseline (week 0) and 3 weeks (week 9) after the third immunization. The number of IFN-γ secreting PBMCs from patients that were specific for Ad5 was determined by ELISpot. Both the Ad5 NAb titers and Ad5 CMI responses were significantly elevated at week 9 (Mann-Whitney test). Horizontal line and error bar indicate the mean ± SEM.

Figure 3

Correlation between pre-existing Ad5 NAb activity and highest levels of induced CEA CMI responses (A). Correlation between vector induced Ad5 NAb activity and CEA CMI responses (B). The r2 values revealed no correlation between pre-existing or vector induced Ad5 NAb activity and CEA CMI ELISpot responses.

Figure 4. Kaplan-Meier survival plots of patients treated with Ad5 [E1-, E2b-]-CEA(6D)

Patients treated at least two times with Ad5 [E1-, E2b-]-CEA(6D) were followed for survival. Panel A represents 6 patients in cohorts 1 and 2 that were followed for survival. There were 4 events in this group. Panel B represents 19 patients in cohort 3 and Ph II that were followed for survival. There were 9 events in this group. Panel C represents all 25 patients (cohorts 1,2, 3 and Ph II) that were followed for survival. There were 13 events in this group.