New Directions in the Treatment of Advanced Colorectal Cancer

ABSTRACT: As the chemotherapy horizons have expanded in colorectal cancer with development of oxaliplatin (Eloxatin) and irinotecan (CPT-11, Camptosar), so too have our approaches to therapy. Numerous immunotherapy and gene therapy approaches are undergoing initial study. These methods are founded on increased knowledge of tumor biology. That same knowledge has led to the identification of new molecular targets for anticancer chemical therapies. This article highlights some of these developments with focus on epidermal growth factor receptor and angiogenesis. [ONCOLOGY 15(Suppl 5):27-30, 2001]

Introduction

During the past decade, the treatment of advanced colorectalcancer finally began to change. The development of new drugs with single-agentactivity in colorectal cancer has changed the focus of clinical trials away fromnew methods of modulation of fluorouracil (5-FU). Currently, new drugdevelopment in colorectal cancer is going beyond traditional cytotoxic therapy.This article is designed to broadly outline some of these new approaches, with afocus on two novel strategies, ie, targeting the epidermal growth factorreceptor (EGFr) and targeting vascular endothelial growth factor (VEGF).

Immunotherapy Approaches

The term "immunotherapy" now encompasses severaldifferent approaches to treatment of malignancies. Reviews of immunotherapyapproaches to colorectal cancer have been published.[1,2]

Using Monoclonal Antibodies

One immunotherapy approach is to use monoclonal antibodiesagainst various cellular targets such as proteins located (and oftenoverexpressed) on the surface of cancer cells. Examples of these includeantibodies to glycoprotein 17-1a, carcinoembryonic antigen (CEA), HER2/neu, andC242.[1] While antibodies themselves may be able to produce an immune responseto aid in tumor cell kill, it is also possible to use antibodies to deliver thechosen anticancer therapy. For example, anti-CEA antibodies have beenradioactively tagged for the purpose of immunoscintigraphy. This is currentlybeing evaluated for delivery of intratumoral radiation.[2]

Antibodies can also be used to enhance delivery of chemotherapyagents or their prodrugs to the tumor. The drug can be attached to an antibodytargeted to the cancer. The agent would then be cleaved and activated by thecancercells. Similarly, toxins such as ricin or Pseudomonas toxin have been attachedto an antibody for targeted delivery.[1] Trials of several such agents havebegun to accrue patients.

Using Vaccines

Another approach in early clinical testing is the use ofvaccines against proteins that are either overexpressed or mutated in colorectalcancer cells. The immune response in this case may require specific humanleukocyte antigen (HLA) types as is the case with the anti-ras vaccine developedby Carbone et al, which is currently undergoing evaluation in colorectal cancer.In addition, other approaches such as adding cytokines may increase theprojected immune response to a vaccine. One such trial combined a monoclonalantibody to glycoprotein 17-1a with interleukin-2 and granulocyte-macrophagecolony-stimulating factor (GM-CSF [Leukine]).[3] Of 20 patients enrolled, onehad a partial response and two had stable disease.

While these approaches have not yet demonstrated efficacy inadvanced disease, the trials are still early and results are limited. However,it is suspected that immune therapies will be most effective in the setting ofminimal residual disease. For example, despite minimal activity in advanceddisease, the monoclonal antibody to glycoprotein 17-1a, in one trial, appears tohave reduced the risk of recurrence by 30% in patients with resected stage IIIcolon carcinomas.[1]

Gene Therapy

Gene therapy is a broad area of current investigation with avariety of potential applications. An excellent review of gene therapy waspublished previously.[4] Some of the classifications and potential targets ofgene therapy are listed in Table 1.

Viral Vectors

Recent advances in our understanding of the biology of cancerhave focused on the genetic changes that lead to malignant transformation. Genescommonly mutated in colorectal cancer include, but are not limited to, p53, theras family, DCC, APC, and genetic mutations that lead to microsatelliteinstability. If mutations in these genes can lead to cancer, then replacing thewild type genes theoretically may reverse the malignant process created by themutant gene.[4]

Therefore, there has been much focus on development of viralvectors to "correct" the mutant gene. For example, by replacing wildtype p53, it is hoped that tumor cells would more readily undergo apoptosis, theprocess of programmed cell death. The adenovirus Onyx-015 utilizes a differentapproach to gene therapy. Already in clinical testing, this virus replicatesselectively in cells with mutated p53, which results in cell death. Evidence forclinical activity was seen in the first trials of intratumoral injection of theOnyx-015 adenovirus.

Immunomodulation

A method of gene therapy that crosses into the previouslydiscussed category of immunotherapy is immunomodulation. One example is use of aviral vector to increase HLA B7 expression to enhance immune responses tomalignancy. Another example is the transfer of cytokine genes into tumor cellsin order to enhance immune response. These methods are being studied in thepreclinical setting and in early clinical trials.

Transferring Genes

Finally, it may be possible to kill tumor cells by transferringgenes that result in the production of certain enzymes that metabolize prodrugsto active cytotoxic agents.[4] For example, by transferring the gene forthymidine kinase into the tumor cells, the antiviral agent ganciclovir(Cytovene) would then be selectively activated in the cancer cells resulting inregional toxicity without severe systemic effects. Other enzyme/prodrug pairsare listed in Table 1.

Molecular Targets

The new knowledge has also resulted in the development ofchemicals and antibodies to molecular targets within tumor cells or the matrixwithin which they grow. Two molecular targets have entered clinical trials incolorectal cancer.

Epidermal Growth Factor Receptor

The epidermal growth factor receptor (EGFr), also known as HER1and ErbB1, is a 170-kD transmembrane glycoprotein consisting of two major parts:an extracellular ligand-binding domain and an intracellular tyrosine kinasedomain. Although EGF is a ligand for EGFr, other ligands have been identified,including transforming growth factor-alpha (TGF-alpha), heparin-binding EGF,amphiregulin, and betacellulin. When an activating ligand binds to EGFr, eitherhomodimerization of EGFr or heterodimerization with another member of the HERfamily occurs. This results in receptor autophosphorylation and activation ofthe tyrosine kinase. The downstream effects of activating the tyrosine kinaseappear to include cell survival and apoptosis, angiogenesis, cell motility, andtumor invasiveness.

The EGFr has been studied specifically in colorectal cancer.EGFr is frequently overexpressed in colorectal cancer. In addition, results ofat least one trial suggest that EGFr overexpression may correlate withmetastatic potential in colorectal cancer.[5]

Drugs that target one of the two major domains of the EGFr havebeen designed and are undergoing clinical testing. One approach to inhibitingEGFr through the extracellular domain is via the creation of antibodies to theextracellular domain of EGFr. Conjugating a toxin such as ricin to either anEGFr ligand or to an EGFr antibody can also target EGFr. Finally, antisenseoligonucleotides to EGF receptor mRNA and to EGF receptor ligand mRNA have alsobeen designed. Of these methods, antibodies to the EGFr have already enteredclinical trials. The antibody ABX-EGF is a fully human IgG2 monoclonal antibodywith a high affinity for EGFr.[6] ABX-EGF had significant activity against avariety of cell lines in the preclinical setting. Phase I trials are ongoing.

A second antibody, C225, is a chimeric antibody against EGFr.Although it is not fully human, C225 does not appear to induce antichimericantibodies.[7] In an early trial, C225 was combined with chemotherapy forpatients with refractory tumors.[8] Although the study reported on very fewpatients treated with a variety of agents, one patient with colorectal cancerhas had a durable complete response. Therefore, a phase II study was initiatedusing irinotecan (CPT-11, Camptosar) and C225 for patients withirinotecan-refractory colorectal cancer; preliminary results are to be presentedat the American Society of Clinical Oncology Conference in May 2001.

Inhibiting the Tyrosine Kinase Domain

Chemicals have also been discovered and/or designed to inhibitthe tyrosine kinase domain, eg the pyrimidine family, which includespyridopyrimidine (PD 158780) and pyrrolopyrimidine (CGP 59326A) and thequinazoline family, which includes several compounds such as ZD-1839 and PD153035. ZD-1839 is an oral agent that has already completed phase I studies. Theagent was well tolerated when administered in an intermittent schedule, andclinical activity was seen in a range of tumor types.[9] ZD-1839 was also welltolerated as a continuous schedule, with several patients achieving stabledisease.[10] Disease-specific studies are pending.

Another member of the quinazoline family, CP-358,774, has beenadministered orally in a variety of schedules in two separate trials.[11,12]While target drug concentrations were achieved at tolerable doses, the maximumtolerated dose had not been achieved in either study.

Angiogenesis

The process of angiogenesis, or new blood vessel formation, hasreceived a great deal of attention in the medical literature and lay press.Angiogenesis is a complex process with numerous potential targets for new drugdevelopment. This paper will focus on agents that target just one aspect ofangiogenesis.

Vascular endothelial growth factor (VEGF), as its name suggests,binds to the VEGF receptor resulting in vascular endothelial cell proliferation.VEGF can be inhibited by binding the VEGF molecule and preventing VEGF bindingto the receptor, or by inhibiting its receptor, Flk-1.

SU-5416

SU-5416 is a novel compound capable of inhibiting the tyrosinekinase portion of the Flk-1 receptor. It has been tested in two phase Itrials.[13,14] In the first trial, 63 patients were treated with SU-5416 twiceweekly. Dose-limiting toxicities were headache and projectile vomiting, whichwere reversible within 48 hours.[13] Other mild toxicities included phlebitisand alterations in transaminase levels. Twice-weekly administration at the phaseII dose of 145 mg/m² resulted in serum SU-5416 levels adequate to inhibit tumorgrowth in animal studies.[15] (Levels were measured by 24-hour AUC [area underthe concentration-time curve].) In a more recently reported trial of thetwice-weekly schedule in 21 patients, the dose had reached 145 mg/m² butdose-limiting toxicities were not yet reported.[14] Headache and emesis appearedto be dose related.

In another recent phase I trial, SU-5416 was combined with 5-FUand leucovorin using two schedules for 5-FU/leucovorin with twice-weeklySU-5416.[16] Both regimens were administered with full-dose (145 mg/m²) SU-5416.The weekly "Roswell Park" regimen was more tolerable than the daily ×5 "Mayo Clinic" schedule of 5-FU/leucovorin. Toxicities on this studywere predominantly those associated with 5-FU and leucovorin therapy. Of 19patients evaluable for response, one had a complete response, five had partialresponses, and nine had stable disease. At the time of the report, mediansurvival and time to progression had not yet been reached and 21 patientsremained on treatment.

Because it was associated with lower rates of grade 3/4toxicities, the Roswell Park regimen was chosen for a randomized trial of5-FU/leucovorin with or without SU-5416. The addition of 5-FU and leucovorin didnot appear to alter the pharmacokinetics of SU-5416. Other receptor inhibitorsincluding an oral formulation, SU-6668, are also being developed.

RhuMAb VEGF

A recombinant human monoclonal antibody to VEGF (RhuMAb VEGF)was also studied in a phase I trial in which 25 patients were treated at fivedose levels ranging from 0.1 to 10 mg/kg administered over 90 minutes.[17] Nograde 3/4 toxicities were observed, but three episodes of tumor-related bleedingoccurred. Headache, asthenia, fever, arthralgias, and rash were among the mildtoxicities observed. The antibody demonstrated linear pharmacokinetics with aterminal half-life of 17 days. No patient developed anti-RhuMAb VEGF antibodiesand it appeared that VEGF concentrations decreased with RhuMAb VEGF. Subsequenttrials were conducted in breast, colon, lung, and prostate cancers.

The colon cancer trial was a randomized phase II trial comparing5-FU/leucovorin plus RhuMAb VEGF at one of two different doses to5-FU/leucovorin.[18] Because crossover to RhuMAb VEGF was allowed for patientsin the 5-FU/leucovorin control arm, primary end points were time to diseaseprogression and response rates rather than survival. There was an imbalance inthat more male patients were in the control arm. Table 2 shows response ratesand time to progression as measured by an independent review facility. Thesedata suggest that the addition of RhuMAb VEGF to chemotherapy may increase timeto disease progression in patients with colorectal cancer. Phase III analysisusing RhuMAb VEGF is pending.

Similar results were found in a trial of RhuMAb VEGF inconjunction with chemotherapy in lung cancer patients.[19] However, that studyhad six patients in the RhuMAb VEGF arms who had life-threatening hemoptysis,with four deaths. It is unclear if this is a drug-related toxicity or acomplication of the disease.

Conclusion

This review partially lists the new targets and agents beingtested in colorectal cancer. Even if only a fraction of these agents arebeneficial, the treatment of colorectal cancer is likely to change significantlyover the next several years. In addition to the potential for new treatmentoptions, optimal administration of older drugs may soon be better refined.Laboratory correlative data show that tumor expression of enzymes such asthymidine phosphorylase, thymidylate synthase, and dihydropyrimidinedehydrogenase may predict 5-FU efficacy. It is now time to incorporateprospective assessment of such predictive markers into clinical trials to bettertarget chemotherapy for colorectal cancer patients.

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