As already mentioned, p185HER2 represents a potential target for tumors that overexpress HER2. A novel approach that is currently being explored is the combined use of therapies directed at p185HER2, such as MoAbs given alone or in combination with conventional chemotherapeutic agents, including paclitaxel(Drug information on paclitaxel).
Several groups have produced antibodies directed against the p185HER2 receptor protein on human cells. Some of these antibodies can inhibit the growth of monolayer cultures of breast and ovarian tumor cells that overexpress p185HER2.[15-18,20,34,35] In extensive studies conducted at Genentech, Inc, a clear relationship between the level of HER2 proto-oncogene expression and sensitivity to the growth-inhibitory effects of the antibodies was observed.[20,21,34]
Breast carcinoma cells with little expression of p185HER2 (eg, MCF7, MDA-MB-231, ZR-75-1, and MDA-MB-436) were not inhibited by MoAbs. Cell lines with higher levels of p185HER2 (MDA-MB-175, MDA-MB-453, MDA-MB-361) were increasingly more sensitive to antibody-mediated growth inhibition. SK-BR-3 and BT-474, the highest expressors of p185HER2 of the cell lines studied, were the most sensitive to the antiproliferative effects (showing approximately 70% growth inhibition).
The most potent growth inhibitory anti-p185HER-2 MoAb was 4D5. The activity of MoAb 4D5 and a humanized version of this MoAb against human breast adenocarcinoma cells bearing HER2/c-erbB-2 has been evaluated in the nude mouse xenograft model. Inhibition of tumor growth has been observed[36,37] with eradication of well-established tumors. Thus, MoAb 4D5 appears to have potential therapeutic applications for tumors overexpressing p185HER2.
A "Humanized" Antibody
In an attempt to circumvent an antiglobulin response during therapy, a "humanized" antibody was constructed by Genentech scientists. Denoted rhuMoAb HER2, this antibody contained the antigen-binding portions of murine MoAb 4D5 (discussed above) and a human immunoglobulin variable region framework. rhuMoAb HER2 has potency comparable to murine 4D5 in blocking the proliferation of breast carcinoma cells in vitro. Furthermore, rhuMoAb HER2 IgG1 is much more efficient in supporting antibody-dependent cellular cytotoxicity, which could increase its antitumor activity.
Initial phase I studies were conducted, and a phase II study has recently been completed in patients with metastatic breast carcinomas overexpressing HER2. Forty-six patients were treated with a loading dose of 250 mg of IV rhuMoAb HER2, then with 10 weekly doses of 100 mg each. Patients with no progression of disease at the completion of this treatment period were offered a weekly maintenance phase of 100 mg.
The study patients had extensive metastatic disease, and most had received extensive prior anticancer therapy. Adequate serum levels of rhuMoAb HER2 were obtained in 90% of the patients. The mean serum half-life of rhuMoAb HER2 was 8.3 ± 5.0 days.
Interestingly, rhuMoAb HER2 serum half-life was found to be dependent on the presence of circulating p185HER2 receptor extracellular domain ECDHER2 released from the tumor into the serum. In those patients with circulating levels of tumor-shed ECDHER2 in serum that were more than 500 ng/mL, rhuMoAb HER serum half-lives were shorter, and therapeutic trough levels of rhuMoAb HER2 were not achieved. A likely explanation for these observations is that, in the presence of ECDHER2 in the serum, antigen-antibody complexes form and are rapidly cleared from the circulation.
Toxicity was minimal, and no antibodies against rhuMoAb HER2 were detected in any patient. Objective responses were seen in 5 of the 43 evaluable patients: 1 complete remission and 4 partial remissions (overall response rate, 11.6%; 95% CI, 4.36% to 25.9%) Responses were observed in liver, mediastinum, lymph nodes, and chest wall lesions. Minor responses, seen in 2 patients, and stable disease, occurring in 14 patients, lasted for a median of 5.1 months. One patient is still in pathologically confirmed complete remission 72 months after starting therapy.
Thus, rhuMoAb HER2 is clinically active in patients who have metastatic breast cancers that overexpress HER2 and have received extensive prior therapy. A confirmatory study that will include 200 patients with less heavily pretreated metastatic disease is currently underway.
Anti-HER MoAb Combined With Chemotherapy
In order to enhance the antitumor activity of anti-p185HER2 MoAb, several investigators have used these antibodies in combination with chemotherapeutic agents. Hancock et al used the MoAb TAB 250, specific to an extracellular epitope of the p185HER2 protein, in combination with cisplatin(Drug information on cisplatin) (Platinol) against human breast carcinoma SKBR-3 cells and ovarian carcinoma SKOV-3 cells. They showed that MoAb TAB 250 markedly enhanced the antitumor effects of cisplatin both in vitro and in vivo.
Using the same antibody, Arteaga has shown enhanced etoposide(Drug information on etoposide) (VePesid)-induced cytotoxicity against human breast carcinoma cells and postulated that p185HER2 antibodies may alter the sensitivity of topoisomerase II toward etoposide. Slamon, in studies using MoAb 4D5, has demonstrated that 4D5 enhances sensitivity to cisplatin in cisplatin-resistant ovarian carcinoma cell lines. A possible mechanism is 4D5's interference with the repair of cisplatin-induced DNA damage, thus promoting cisplatin cytotoxicity.
Paclitaxel/Doxorubicin/ rhuMoab HER2 Regimen
We have explored the activity of paclitaxel and doxorubicin(Drug information on doxorubicin), the two chemotherapeutic agents most active against breast cancer, in combination with rhuMoAb HER2. Studies of the combined therapy with 4D5 were conducted in monolayer cell culture soft agar, as well as with breast cancer human tumor xenografts in nude mice. For the in vivo studies, 107 BT-474 cells, which express high levels of p185HER2, were grown subcutaneously to a mean size of 200 mm³ over 11 days. Animals were then treated with MoAb 4D5 alone at a dose of .3 mg/kg intraperitoneally (IP) twice a week, paclitaxel alone at a dose of 20 mg/kg IV, or both therapies combined.
Therapy with MoAb 4D5 alone produced a 35% growth inhibition, and paclitaxel alone resulted in a 35% growth inhibition when compared with animals treated with a control MoAb. The treatment with paclitaxel plus 4D5 resulted in major antitumor activity, with 93% inhibition of growth. This result was markedly better than an equipotent dose of doxorubicin (10 mg/kg IP) and 4D5 (70% inhibition). In addition, paclitaxel combined with 4D5 resulted in the disappearance of well-established xenografts.
Cisplatin/rhuMoAb HER2 Therapy
In parallel with the phase II clinical trial described above, which used rhuMoAb HER2 alone, a phase II study of rhuMoAb HER2 in combination with cisplatin has been conducted in patients with breast carcinomas that overexpress p185HER2 and a history of proven refractoriness to chemotherapy. Thirty-six patients were treated with an IV loading dose of 250 mg of rhuMoAb HER2 on day 0, followed by weekly administration of 100 mg of MoAb in combination with cisplatin, 75 mg/m², on day 1 and every 3 weeks thereafter.
In this group of patients with expected cisplatin resistance, the observed response rate to the combined therapy was 25%, suggesting that the synergy observed in the laboratory was reproducible in the clinic. In addition, the combined therapy was no more toxic than cisplatin alone.
Phase III Study of rhuMoAB HER2 Combined With Chemo
Results from the phase II studies and the activity of rhuMoAb HER2 against xenografts when given in combination with doxorubicin and paclitaxel have been encouraging. These positive results have led to the design of a phase III multinational study of chemotherapy in combination with rhuMoAb HER2 in patients with HER2-overexpressing breast tumors who have not received prior chemotherapy for metastatic disease (Figure 2). In addition to proven HER2 overexpression by immunohistochemistry, eligible patients must have measurable disease and must not have received prior chemotherapy for metastatic disease.
In this ongoing study, patients are randomized to one of two treatment arms: the active arm, which consists of rhuMoAb HER2 in combination with cytotoxic chemotherapy; or the control arm, which consists of cytotoxic chemotherapy alone. Patients who are randomized to the rhuMoAb HER2 arm receive weekly administration of the antibody at a dose similar to the phase II studies. After the completion of cytotoxic chemotherapy, patients assigned to the HER2 arm continue with weekly rhuMoAb HER2 administration until disease progression. Patients in the control arm are given the option of receiving rhuMoAb HER2 when disease progression occurs.
Patients receive one of two chemotherapy regimens for a minimum of six cycles: cyclophosphamide(Drug information on cyclophosphamide) and doxorubicin or epirubicin(Drug information on epirubicin), if patients have not received anthracycline therapy in the adjuvant setting; or paclitaxel, if patients have received anthracycline therapy in the adjuvant setting. Because anthracyclines are widely used in the adjuvant setting, it is likely that a significant number of patients will be treated with paclitaxel ± rhuMoAb HER2. The main goal of this study is to determine whether the addition of this anti-HER2 antibody increases the time to disease progression compared with the group of patients treated with antibody alone.
An alternative approach to interfering with the function of type I tyrosine kinase receptors , including p185HER2, is direct inhibition of the receptor protein tyrosine kinase. These enzymes catalyze the transfer of the terminal phosphate from adenosine(Drug information on adenosine) triphosphate (ATP) to the phenolic group of tyrosine in substrate proteins. Erbstatin, synthetic compounds known as tyrphostins, and the 2-thio-indoles inhibit the tyrosine kinase activity of the type I receptors by competing with ATP.[43-45] Once the activity of these agents has been defined, a logical next step would be to combine them with paclitaxel in vitro and in vivo.
To explore the potential anticancer effects of these compounds, we have studied the 2-thio-indole, PD153035, a reversible inhibitor of the EGF receptor tyrosine kinase and, to a lesser degree, of the p185HER2 receptor. PD15035 completely inhibited phosphorylation of the EGF receptor and p185HER2 tyrosine kinase and prevented anchorage-dependent and -independent growth. In addition, a close correlation was observed between higher receptor number and greater growth inhibition.
It is likely that these agents may be able to increase the antitumor activity of paclitaxel in a similar fashion to that observed with anti-HER2 antibodies. Support for this concept is provided by M-C Hung and colleagues in their studies with emodin, a protein tyrosine kinase inhibitor that inhibits p185HER2 tyrosine kinase activity. In non-small-cell lung cancer cells that overexpress HER2, they observed a marked synergism of emodin given in combination with chemotherapy.
Overexpression of p185HER2 results in activation of a series of signaling pathways, including the ras/raf/MAP-kinase pathway. Peptidomimetic inhibitors of farnesyl:protein transferase that selectively block farnesylation of cellular proteins, including p21ras, inhibit ligand-induced stimulation of MAP-kinase in breast cancer cells and also have the potential for combination with paclitaxel in preclinical models.
The predictive value of HER2 overexpression for paclitaxel response requires independent confirmation in advanced disease and early breast cancer. This question could be addressed in ongoing studies of paclitaxel-based adjuvant therapy in breast cancer. In preclinical models, the combined therapy of breast cancer cells that overexpress HER2 with agents that interfere with HER2 function and paclitaxel results in a marked antitumor effect. One such strategy, the combination of anti-HER2 MoAb with paclitaxel, is currently being evaluated. If the results of these studies are positive, we might be faced with a novel paradox, in which expression of a receptor that confers a worse prognosis provides us with an opportunity for increased response to taxanes.