The goal of this clinical investigation was to evaluate the safety and the biologic activity of treating cancer patients with multiple, intravenous doses of CP-870,893, a fully human agonist mAb specific for the cell-surface molecule CD40. We show here that weekly dosing schedule of CP-870,893 is feasible, associated with cytokine release syndrome which defines the MTD, but was not associated with objective clinical responses. Correlative immune assessment studies suggest that administration of CP-870,893 on a weekly schedule led to persistently increased expression of costimulatory and adhesion molecules of B cells without resetting to baseline. Additionally, in up to 50% of patients, there were potentially deleterious consequences on T-cell populations as evidenced by marked declines in the absolute count of total CD3+ T cells and both the CD4 and CD8 T-cell subsets. In the previous trial of single-dose CP-870,893 infusion conducted in a similar group of patients, chronic B-cell stimulation and T cell depletion were not observed. These findings have important implications for future trials exploring CP-870,893 as an immunomodulatory agent in cancer patients and suggest that an interval of longer than one week may be desirable for optimal immune pharmacodynamics.
There was a high degree of similarity in the toxicology and pharmacology of weekly dose CP-870,893 evaluated in this study and that of single-infusion dosing previously reported.13
As in the single-dose trial, the MTD for weekly infusion of CP-870,893 was estimated at 0.2 mg/kg, suggesting that the administration of multiple doses at weekly intervals did not significantly increase toxicity of this drug. Good tolerability of the weekly dosing schedule was further validated by the small fraction of scheduled infusions (2.7%) that were delayed due to toxicities. CRS was again observed to be the most common adverse event. Grade 3 CRS constituted two out of three observed DLTs. Transient and mild-to-moderate elevations of liver function tests (AST, ALT, bilirubin) and D-dimer with each CP-870,893 infusion were observed as were transient declines in lymphocytes, monocytes and platelets. The changes in laboratory abnormalities most likely reflect CP-870,893 interaction with CD40-expressing lymphocytes, monocytes, platelets and endothelial cells. As with the clinical adverse events, these changes in laboratory test results were very similar in nature and grade to those observed following a single infusion of CP-870,893.13
Only one immune-mediated toxicity was observed and diagnosed as autoimmune diabetes, which was most likely an exacerbation of an underlying condition as the patient had measurable anti-islet cell antibodies at baseline that increased during 16 weeks of therapy, associated with a new insulin requirement. Importantly, immune mediated events such as those classically observed for blocking anti-CTLA-4 mAb,16
such as colitis, dermatitis and hypophysitis, were not observed in this or previous studies of CP-870,893. Serum half-life of CP-870,893 and other pharmacokinetic parameters were also very similar between weekly dose and single dose. No human anti-human antibodies have been identified for weekly dosing or single dosing of CP-870,893.
The best clinical response in this trial was stable disease (SD) in 26% of patients. There were no objective clinical responses (PR or CR), unlike the single-dose study in which 4 of 27 patients (26%) had a PR (all responders had melanoma).13
Although it is impossible to compare objective response rates between these two small studies of single-dose vs. weekly CP-870,893, it was disappointing not to be able to confirm some level of objective antitumor activity in this study, especially considering that the eligibility criteria and two investigational sites were otherwise the same and that melanoma patients were also represented in the weekly dose clinical trial.
A prominent pharmacodynamic effect of CP-870,893 treatment in vivo is depletion and activation of peripheral blood B cells,13
which we have also recently modeled in vitro.10
Here, we found that B cells rapidly but transiently decreased in the circulation with each weekly dose, similar to our observations in patients treated with a single infusion, but returned to baseline both with respect to percent of lymphocytes and absolute counts on the day of subsequent infusions. As expected, B cells isolated one or two days after infusion in this study exhibited increased expression of CD86 and CD54, consistent with activation, and importantly this effect was observed with each dose. However, expression of CD54 or CD86 B cells did not reset to baseline by the time of subsequent infusions. In the single dose trial, B cell activation parameters always eventually reset to baseline,13
and presumably would have also reset in this study except that CP-870,893 was re-administered once per week. At the time of the fourth infusion of CP-870,893, for example, the percentage of B cells expressing CD86 was 3-fold higher compared to baseline, despite the fact that the absolute number of B cells was the same. The pharmacodynamic effect of B cell activation may or may not represent a mechanism of action of CP-870,893. In vitro, B cells that are activated via CD40 in general,17
and via CP-870,893 in particular,10
act as potent antigen presenting cells and in certain model systems, drive the induction of the antitumor T cells.18–20
Whether or not such a mechanism occurs in vivo in response to CP-870,893 remains to be determined. More classically, the immunological power of CD40 activation has been linked to activation of dendritic cells,21
with potent T-cell responses being observed even in the absence of B cells;4
however, recent data underscore the role of B cells and their potential function as APC in regulating T cell function.22
Regardless of the role of B cells in the mechanism of action of CP-870,893, our findings that peripheral B cells appear persistently activated with weekly CP-870,893 infusion raise the possibility of chronic, even global, immune stimulation induced with the antibody used on this schedule. Such chronic B-cell activation was not observed with a single dose of CP-870,893.13
Our observations that weekly CP-870,893 may have induced chronic immune stimulation are concerning in light of published results from mouse models suggesting that overstimulation with CD40 agonists may be deleterious to adaptive immune responses.14,15,23
Similar to many of the patients treated with weekly CP-870,893 reported here, mice given frequent or high doses of anti-CD40 mAb exhibit T-cell depletion. For example, in an implantable model of melanoma, weekly injection of an agonistic anti-CD40 antibody into tumor-bearing mice accelerated the deletion of tumor-antigen specific T cells, even though an initial expansion of antigen-specific T cells was seen after the first dose of antibody.14
In another study, mice injected with repeated daily doses of an agonist anti-CD40 mAb (with IL-2 or IL-15) exhibited marked CD4+
Tumor-bearing mice treated with anti-CD40 mAb and IL-2 were initially protected but were unable to mount effective memory responses and succumbed to secondary tumor challenge.15
Similarly, anti-CD40 mAb resulted in severe depression of the CD8+
T-cell response to challenge with lymphocytic choriomeningitis virus.23
Indeed, high daily dosing of anti-CD40 mAb has been used to control the autoimmune inflammatory process associated with chronic collagen-induced arthritis,24
emphasizing the potential immunosuppressive quality of certain dose and schedule regimens of anti-CD40 mAb. Thus, although many important issues distinguish these mouse models from our clinical trial, our findings raise the hypothesis that too frequent dosing of CP-870,893 may result in counterproductive immune modulation manifesting as persistent B-cell stimulation and in some patients, peripheral blood T-cell depletion. In patients receiving a single dose of CP-870,893 at similar doses, marked T-cell depletion in the peripheral blood was not observed at the end of study, and no patients in this previous study at the MTD or higher had CD4+
T-cell counts <200 cells/µl at the end of study. Still, we cannot exclude the possibility that T-cell depletion observed at the end of study in some patients was associated with tumor progression. We speculate that the lack of objective tumor responses observed with weekly infusion of CP-870,893—in contrast to the responses seen in the single dose study—may be related to counterproductive immune hyperstimulation with short-interval dosing. On the other hand, it is important to keep in mind that CD40 agonists may also mediate antitumor effects directly (non-immunologically). Certain tumors express high levels of CD40 and direct activation of CD40 results in growth inhibition and sensitization to cytotoxic agents. CP-870,893, for example, has been reported to have antitumor effects in murine systems for which lymphocytes are absent.12
Our observations here of stable disease in patients with renal cell carcinoma and mesothelioma may be important in this regard, but we did not have the opportunity to measure CD40 expression of tumors from patients in this study.
The observed half life of CP-870,893 of less than 6 hrs is unusually short for a fully human IgG2 molecule. This finding was similar to the CP-870,893 half life observed in the single dose study and was one of the reasons that a relatively short dosing interval of one week was chosen for a repeat dose study. The reasons for this short half life remain unknown, but it has been speculated that it reflects a large sink of CD40 (receptor) molecules in vivo. When given to mice, the half life of CP-870,893 is about 3 weeks. For another agonist CD40 mAb SGN-40 (which is IgG1 not IgG2), initial pharmacokinetic data in humans demonstrated that the half-life was also unusually short and dependent on the dose given, ranging from 0.9–2.9 days after the first dose.25
The doses of SGN-40 used were higher than we have explored for CP-870,893. It was suggested that for SGN-40 there may be a rapid elimination pathway or redistribution volume that had not been saturated,25
but in a subsequent publication regarding SGN-40, pharmacokinetic parameters could not be estimated because of limited sample collections.26
Current trials of CP-870,893 are utilizing dosing intervals of 3 or 4 weeks to address the potential concern of CD40-mediated overstimulation. Other approaches to improve efficacy include strategies for combination therapy.1
Active approaches are combining CP-870,893 with chemotherapy (e.g., with carboplatin and paclitaxel for patients with advanced solid tumors or with gemcitabine for chemotherapy naive patients with pancreatic cancer) or with a cancer vaccine (e.g., poly IC:LC and NY-ESO-1/gp100/MART-1 peptides emulsified with Montanide ISA 51 for patients with melanoma). Recently a trial has opened combining CP-870,893 every three weeks with the anti-CTLA-4 blocking mAb tremelimumab every 12 weeks for patients with metastatic melanoma. Careful immunological assessments are important endpoints of these clinical trials.
In summary, this phase I study of weekly infusions of the agonistic, fully human CD40 mAb CP-870,893 showed that a weekly dosing schedule was well tolerated and feasible, but there was little antitumor clinical activity of this infusion schedule. Although the reasons for this remain speculative, correlative studies presented here suggest that less frequent administration of CP-870,893 may be advantageous to avoid undesirable immune modulation. It will be important to consider these findings in the design of future trials of this and other agonistic CD40 antibodies.