From targeted to multitargeted cancer treatments
EMBO Member Alexander Levitzki, Professor of Biochemistry at the Hebrew University of Jerusalem, Israel, pioneered the generation of tyrosine phosphorylation inhibitors (tyrphostins) in the late 1980s to the mid-1990s. Work in his group focused on the development of “tyrphostins”, or “tyrosine kinase inhibitors” (TKI), directed against epidermal growth factor receptor (EGFR), Her-2, Bcr-Abl, Jak-2, vascular endothelial growth factor receptor 2 and platelet-derived growth factor receptor. These findings led to the development of 15 tyrosine kinase inhibitors that are currently used in cancer therapy. Dramatic effects have been achieved in the treatment of early chronic myelogenous leukemia with Gleevec. Nevertheless, despite the increasing use of tyrosine kinase inhibitors in the clinic, their performance has been modest against solid tumours.
Most targeted therapies are aimed at one critical oncogenic marker, so it is easy for tumours to develop resistance, especially as tumours are constantly evolving. “We sought strategies that would hit tumours at many targets, since malignant tumours exhibit ever-changing heterogeneity,” says Levitzki. “We began with tumours that overexpress EGFR, which is currently targeted in the clinic by the tyrosine kinase inhibitors Gefitinib, Erlotinib, Lapatinib and by two antibodies, Cetuximab and Panitumumab. These targeted agents exhibit weak efficacies against tumours that overexpress EGFR.” In these tumours, although EGFR is overexpressed, it apparently is not an essential survival factor. Therefore, in the clinic, only a small subset of EGFR-overexpressing tumours responds to EGFR inhibition, and these tumours often acquire resistance rapidly.
“We have converted the overexpression of EGFR, rather than its activity, into the Achilles heel of the tumour,” says Levitzki. This was achieved by using the EGFR as an entry point into the tumour. Levitzki’s group uses an EGFR-targeting vector to specifically deliver synthetic double-stranded RNA, PolyInosine-PolyCytosine (PolyIC), into tumours that overexpress the EGFR. This results in tumour-specific internalization of large amounts of PolyIC. The internalized PolyIC activates several signaling pathways, including protein kinase R and other double-stranded RNA-dependent factors, leading to cell death. In addition, PolyIC induces a “bystander effect,” due to the production of interferon-alpha, interferon-beta and cytokines that recruit immune cells, such as NK and T cells. These immune cells attack all of the tumour cells, including cells that do not overexpress EGFR (Figure 1). Thus, the internalized PolyIC induces the rapid demise of the targeted cell as well as neighboring tumour cells, but spares the more robust non-tumour cells. Indeed, this vector led to the complete regression of disseminated EGFR-overexpressing tumours in mice (1). “This new strategy tackles an important deficiency of targeted therapy, namely its inability to contend with the heterogeneity of malignant tumours,” says Levitzki.
The vector for delivery of PolyIC consists of polyethyleneimine-polyethyleneglycol–ligand, where the ligand can be epidermal growth factor, as in the initial experiments, or any other suitable ligand. “In our most recent studies, we have replaced the homing epidermal growth factor moiety by a ligand that zeros in on Her-2, destroying Her-2 overexpressing breast cancer cells (2), even ones that are resistant to Trastuzumab (Herceptin).” Similar vectors have been generated to target metastatic prostate cancer using a vector targeting prostate surface membrane antigen (PSMA), and metastatic melanoma using a vector targeting protease activated receptor 1 (Par1).
Another approach to enhance targeted therapy was discovered by serendipity. “We began looking for insulin-like growth factor 1 receptor kinase inhibitors in 1997. After developing a number of generations of such inhibitors, we came across a family of tyrphostins that act as allosteric kinase inhibitors of these receptors,” says Levitzki. An unexpected property of this particular family of novel tyrphostins, which include NT157, was the ability to induce the irreversible proteolytic destruction of Irs1 and Irs2, the signal transducers of insulin-like growth factor 1 receptor. This results in dramatic anti-tumour effects in experimental animals harboring prostate cancer, ovarian cancer or metastatic melanoma (3). Recently, B-Raf inhibitors have been hailed as effective therapy against metastatic melanoma, but resistance develops rapidly. NT157 is effective even against tumours that carry B-Raf activating mutations that are resistant to Vemurafenib (Zelboraf). “Our laboratory is currently developing strategies to induce the irreversible destruction of other signaling molecules, such as mutated K-Ras,” adds Levitzki. The focus of Levitzki’s laboratory is finding ways to target specific tumour markers, while invoking a holistic anti-tumour response.