General features of therapeutic antibodies
Antibody-based therapeutic agents are the most rapidly expanding class of pharmaceuticals.26 Early therapeutic antibodies originated from mice. To reduce immunogenicity, mouse antibody amino acid sequences must be grafted onto a human antibody scaffold.27 Such engineered MoAbs may be either chimeric or humanized.28 Chimeric and humanized antibodies contain 65–90% and approximately 95% human sequences, respectively.27 Transgenic mice and phage-display technologies now permit the de novo generation of fully human antibodies.26
Therapeutic antibodies may be classified further by antibody isotype. Antibodies of the immunoglobulin G1 (IgG1) or IgG3 isotype26 and murine IgG2a antibodies28 potentially support the immune system effector functions of antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC).29 ADCC involves interactions between tumor-bound MoAb and Fcγ receptors on immune cells, such as neutrophils or macrophages, leading to the cell-mediated lysis or phagocytosis of tumor cells. CDC involves binding of a tumor-bound MoAb with complement component C1q, affording cell lysis and the attraction of effector cells.27 In vivo activation of immune effector responses has been demonstrated for the IgG1 antibodies rituximab,30 trastuzumab,30 and cetuximab.31
Various antibodies directed against EGFR are available currently the clinic. These are reviewed in detail below.
Cetuximab (Erbitux®; ImClone Systems Inc., Princeton, NJ) is a human:murine, chimeric anti-EGFR IgG1 MoAb that is indicated for use in combination with irinotecan for the treatment of patients with mCRC who have EGFR-expressing tumors that are refractory to irinotecan-based therapy or as monotherapy in irinotecan-intolerant patients with mCRC who have EGFR-expressing tumors.32 Mechanisms of action of cetuximab include inhibition of cell cycle progression,33 induction of apoptosis,34, 35 inhibition of angiogenesis,36, 37 and decreased invasion and metastatic spread.38 Cetuximab has been studied in clinical trials as a single agent, in combination with second-line treatment of patients with refractory CRC, and as an addition to front-line combinations such as 5-fluorouracil (5-FU), leucovorin, and oxaliplatin FOLFOX-4.39 Preclinical studies have shown that the antitumor activity of cetuximab is potentiated combined with cisplatin, paclitaxel, doxorubicin, topotecan, gemcitabine, 5-FU, or radiation.40
Although cetuximab is the only EGFR inhibitor that has been approved by the U.S. Food and Drug Administration (FDA) for patients with CRC, its development has been somewhat circuitous. Because of the expectation that many biologics lack efficacy as monotherapy, cetuximab was tested first in combination therapy. An initial observation suggesting that cetuximab had the ability to make irinotecan-resistant patients responsive to irinotecan led to a strategy of adding cetuximab to irinotecan in such patients. Although the benefit of such a strategy was confirmed later, the original study failed to convince the FDA of meaningful efficacy, which led to the succession of clinical trials listed below. Therefore, the activity of cetuximab as monotherapy was demonstrated after its activity in combination was apparent.
Treatment with cetuximab plus irinotecan afforded a 22.5% partial response (PR) rate for a median of 186 days as a second-line agent in patients (n = 120 patients) who had progressive disease and tumors that were refractory to irinotecan and 5-FU. The median survival in those patients was 232 days.41, 42 Single-agent cetuximab afforded a 9% PR rate and 37% stable disease or minor responses in patients with EGFR-expressing tumors (n = 57 patients) who had disease progression on an irinotecan-based regimen.43 The median survival of those patients was 6.4 months. The most frequently reported adverse events (AEs) were acne-like rash and asthenia. Single-agent cetuximab has been compared directly with cetuximab plus irinotecan in patients with EGFR-detectable, irinotecan-refractory mCRC.44 Of 577 patients screened, 474 patients (82.1%) had EGFR-positive tumors, and 329 of those patients met all inclusion criteria and were randomized 2:1 to receive cetuximab plus irinotecan (n = 218 patients) or cetuximab monotherapy (n = 111 patients). Patients who had disease progression on cetuximab monotherapy were allowed to cross over to the cetuximab plus irinotecan arm. Response rates were 17.9% in the combination therapy arm and 10.8% in the monotherapy arm. The median time to disease progression was significantly greater for combination therapy versus monotherapy (4.1 months and 1.5 months, respectively; P < 0.001). The difference in overall survival between the 2 arms was not statistically significant (8.6 months for combination therapy; 6.9 months for monotherapy). However, this result may have been due to the crossover study design: Fifty-six patients in the monotherapy group received additional irinotecan after they had disease progression on cetuximab monotherapy.
In patients with previously untreated mCRC, several studies have evaluated cetuximab added to front-line irinotecan/5-FU/leucovorin (IFL). In a single study, 11 patients (44%) achieved a PR, and 5 patients achieved a minor response.45 The median time to disease progression and the median survival were not reached. Frequently reported AEs were diarrhea, neutropenia, and rash. In another trial, 4 of 13 patients with mCRC achieved a confirmed PR; 1 patient demonstrated stable disease, and information on 7 patients was pending.46 In a European study, 52% of 21 evaluable patients with mCRC achieved a PR, and 43% had stable disease, affording a tumor growth control rate of 95%47 with no unexpected toxicities.
A Phase II study evaluated the activity of single-agent cetuximab in patients with EGFR-detectable mCRC refractory to fluoropyrimidine-based chemotherapy plus irinotecan and in combination with oxaliplatin.48 A total of 248 patients (median age, 59 years; male:female ratio, 55:45 patients; Eastern Cooperative Oncology Group performance status [PS] 0:1, 42:57 patients) were enrolled in that study. Patients were received treatment with cetuximab at standard doses (400 mg/m2 loading dose over 2 hours, then 250 mg/m2 over 1 hour weekly). Commonly encountered AEs were skin rash and fatigue/malaise. One patient experienced a Grade 3 allergic reaction that required discontinuation of study treatment. PRs were observed in 12% of patients. Stable disease was observed in 34% of patients for at least 6 weeks, affording a disease control rate of 46%. Significantly, 9 patients with EGFR-undetectable CRC were enrolled, and 2 of those patients achieved a PR.
Therapeutic antibodies can induce hypersensitivity and other infusion-related reactions. Cetuximab infusion reactions typically are seen after the first dose and may be associated with minor fever, chills, and dyspnea. Hypotension, urticaria, and rapid onset of airway obstruction (Grade 3/4) occur in 3% of patients, although these rarely occur with fatal outcome (< 1 in 1000 episodes). These reactions are considered anaphylactoid and are unlikely to be IgE-mediated. Premedication with an H1 antagonist (e.g., diphenhydramine IV or similar agent) is recommended. Premedication regimens consist of 25–50 mg diphenhydramine HCl but vary with respect to pretreatment interval, administration route, and infusion rate. A suggested regimen consists of 10 mg of liquid cetirizine, 150 mg of ranitidine, and 625 mg of acetaminophen given 30 minutes prior to infusion. Initiating the cetuximab infusion at 2.5 mL per minute for 15 minutes and increasing to a maximum of 5 mL per minute also may be beneficial. There is no evidence to suggest that a test dose (20 mg of cetuximab for 10 minutes) reliably can identify patients who are at risk for severe reactions.
The most common toxicity associated with cetuximab treatment is acneiform rash. Rash occurs in approximately 75% of cetuximab-treated patients and resolves upon discontinuation of treatment. In studies across multiple malignancies, rash intensity predicted increased survival.42, 49 Lack of a maximum tolerated dose for cetuximab and correlation between rash and response suggest the possibility of dose escalation in patients who fail to develop rash at the onset of therapy. Confirming this observation in larger studies and, prospectively, investigating the value of tailoring dosing to maximize efficacy will be required.
Preclinical studies have demonstrated that anti-EGFR agents may have little activity when the level of EGFR expression is below a threshold level.50–52 Consequently, cetuximab is indicated only for the treatment of patients who have tumors that demonstrate EGFR expression,32 typically as determined by immunohistochemical (IHC) assay.53 However, there does not appear to be a relation between EGFR staining intensity and tumor response, and the reported percentages of EGFR-detectable tumors have varied.41, 48 Despite the efforts of the American Joint Committee on Cancer to develop a standardized EGFR IHC detection system54 and the availability of an FDA-approved EGFR test kit,53 grading IHC tests often is ambiguous; and a variety of factors affect the cut-off point for receptor positivity and afford significant variability in interinvestigator scoring.55 It is interesting to note that some patients who were assigned EGFR-nondetectable tumor status had demonstrated benefits from cetuximab therapy.48 Clearly, further study is required.
Several important questions remain regarding the role of cetuximab in the treatment of mCRC. Although cetuximab is approved for the second-line treatment of patients with irinotecan-refractory mCRC, the potential role of this agent in the treatment of patients with disease in earlier stages remains understudied. For example, further evaluation is needed of cetuximab in adjuvant or first-line metastatic settings in combination with various agents, such as oxaliplatin, and in patients with EGFR-negative CRC. To this end, Tabernero et al. recently reported the results of the Phase II ACROBAT trial evaluating the efficacy of full-dose cetuximab combined with full-dose FOLFOX-4 in patients with first-line mCRC.39 An 81% response rate (5% CR, 76% PR) was reported in 42 patients available for analysis. The regimen was well tolerated and warranted testing in a larger group of patients. A Phase III trial (EXPLORE) of FOLFOX plus cetuximab in patients who failed a first-line irinotecan-containing regimen also is under way.56 Particularly promising results have been obtained for cetuximab in the treatment of patients with squamous cell carcinoma of the head and neck (SCCHN).57
Panitumumab (ABX-EGF; Amgen Inc.) is a fully human IgG2 anti-EGFR MoAb that is in clinical testing for a variety of tumors. ABX-EGF was derived from transgenic mice,50 offering potentially slower clearance than mouse-derived MoAbs.6 In a Phase I study, 43 patients with CRC, renal carcinoma, prostate carcinoma, nonsmall cell lung carcinoma (NSCLC), pancreatic carcinoma, and esophageal tumors58 (interim data; n = 15 patients) received ABX-EGF therapy. Two patients (1 patient with esophageal carcinoma and 1 patient with CRC) demonstrated stable disease for 7 months and 4 months, respectively. One patient with prostate carcinoma demonstrated a minor response. Toxicities included acneiform rash. No patient developed an infusion-related reaction, serious AE, or human antihuman antibodies.
Interim results are available from a multicenter Phase II study of ABX-EGF in patients with mCRC who previously failed therapy with a fluoropyrimidine and irinotecan, with oxaliplatin, or with both.59 In total, 148 patients are enrolled in 2 cohorts based on EGFR expression levels. At the time of last follow-up, enrollment was complete. All patients were available for evaluation after 8 weeks; 15 patients (10.1%) had confirmed PRs, and 54 patients (36%) had stable disease, as determined by investigator assessment. Skin rash was the most common AE. One patient had a Grade 3 infusion reaction that may have been related to ABX-EGF.
EMD 72000 (EMD Pharmaceuticals, Durham, NC), which is a humanized IgG1 anti-EGFR MoAb, has completed Phase I clinical testing in EGFR-positive solid tumors. Patients (n = 22 patients) with chemotherapy-refractory esophageal, colorectal, head and neck, and cervical tumors received EMD 72000 weekly.60, 61 Objective response (23%) and stable disease (27%) were demonstrated at all dose levels. AEs included rash (64%). EMD 72000 administered to 22 patients with colon, gastric, or renal tumors demonstrated PRs in 2 patients and a minor response in 1 patient.39 Two patients achieved stable disease. A recently reported, Phase I, pharmacokinetic/pharmacodynamic study showed near-complete EGFR signaling suppression at the 1200-mg dose level.62
Other anti-EGFR MoAbs in development
Other MoAbs directed against EGFR have undergone clinical testing. The humanized MoAb hR3 (CIMYM Inc., Mississauga, Ontario, Canada) has shown activity in SCCHN in a Phase I trial.63 ICR62, which is a rat MoAb, also has demonstrated some effect in Phase I trials in patients with SCCHN and NSCLC64; and a Phase I trial of MDX-447 (Medarex, Inc., Princeton, NJ), which is a bispecific anti-EGFR, anti-CD64 humanized antibody, demonstrated efficacy in patients with solid EGFR-positive tumors.65, 66