REVIEWS (DES ahighly re ing ch GF in ide (CO AI DGF-B well as prima y DES nd n or PDG of th on ch which pDC bited gro R RB. ment wi th Glv and co ative di and de ed TEL FR fusi gs for th as yet un r gen .0t ting the in this partic .by e with th he cen d CE in the regula add ng too higl nical data available so far in CML,GISTand that ing PR C-KI nd into titi.. Is and st dise cell in this tumour model dono early phases of lopment of a molecul aonyclundtrtodmalig capillary endothelial ells,pericytes and smooth-muscl other malignancies that involve any of these signalling 500 JULY 2002 VOLUME 1 2002 Nature Publishing Group © 2002 Nature Publishing Group 500 | JULY 2002 | VOLUME 1 www.nature.com/reviews/drugdisc REVIEWS cells79, but might also be influenced indirectly through paracrine action on PDGF-responsive stromal and perivascular cells, which are a principal source of vascu￾lar endothelial growth factor (VEGF)80. PDGF has also been shown to induce the expression of VEGF in endothelial cells, which in turn causes an autocrine VEGF loop81.Anti-angiogenic activity of Glivec has been shown in vitro through inhibition of serum-stimulated capillary sprouting from rat aorta, and in vivo in a subcu￾taneous implant model in which the drug inhibited PDGF- and also VEGF- and basic fibroblast growth factor (bFGF)-stimulated vascularization82. Blockade of PDGFR signalling by Glivec has also been shown to inhibit angio￾genesis and tumour growth in an experimental model of bone metastasis83. Glivec treatment of nude mice injected with PC-3MM human prostate-cancer cells into the tibia inhibited tumour-cell growth and induced apoptosis, both in tumour cells and tumour-associated endothelial cells. The effects were pronounced when mice were treated with the combination of Glivec and taxol. Interestingly, immunohistochemical studies showed that tumour cells growing in the bone (but not those in surrounding musculature) expressed high levels of PDGF-α, PDGF-β, PDGFR-α and PDGFR-β. Tumour￾associated endothelial cells within the bone also expressed PDGFR-α and PDGFR-β. These data indi￾cate that inhibition of the PDGFR in combination with chemotherapy might provide a new approach for the treatment of bone metastasis. Conclusion The discovery and development of Glivec has shown that is possible to produce rationally designed, mole￾cular-targeted drugs for the treatment of a specific cancer. The research programme has also clearly shown that it is possible to define in vitro and animal models with high predictive quality, as the results of the subsequent clinical studies have largely corrobo￾rated the preclinical findings. The predictive quality was achieved in this particular case by using models with identical genetic abnormalities as those found in man. The case of Glivec also shows that compounds that do not only affect one, but two or more targets (which is frequently the case), can be beneficial in allowing several diseases with differing molecular abnormalities to be addressed, without paying too high a price in terms of toxicity. The clinical data available so far in CML, GIST and chronic myeloproliferative disorders that involve rearrangement of the PDGFR gene indicate that the inhibition of BCR–ABL, c-KIT and PDGFRs can be achieved with Glivec in humans, and translated into clinically meaningful patient benefit. Providing clinical ‘proof of concept’, these data validate the initial hypothe￾sis of this programme, and underscore the importance of rationally selecting the target diseases to be consid￾ered in the early phases of development of a molecule such as Glivec. Beyond these reasonably well-understood malig￾nancies, Glivec could have potential in the treatment of other malignancies that involve any of these signalling Autocrine PDGFR activation is also well docu￾mented in tumour cells of dermatofibrosarcoma pro￾tuberans (DFSP), a highly recurrent, infiltrative skin tumour that is characterized by a chromosomal rearrangement involving chromosomes 17 and 22. The resulting fusion-gene product collagen I, α1 polypep￾tide (COL1A1)–PDGF-β triggers the autocrine stimu￾lation of the PDGFR67. COL1A1–PDGFβ-transformed fibroblasts, as well as primary DFSP and giant-cell fibrosarcoma cell cultures, were inhibited by Glivec in vitro and in vivo 67–69. The main mechanism by which Glivec affected DFSP tumour growth was through induction of apoptosis69. Preliminary data indicate that Glivec might also be active in patients with DFSP70. Relatively little is known about the ligand-indepen￾dent activation of PDGFR. However, rearrangement of PDGFRβ has been described in chronic myeloprolifer￾ative diseases. The best known of these is the t(5;12) chromosomal translocation in chronic myelomono￾cytic leukaemia (CMML), in which PDGFRβ, which is located on chromosome 5, is fused to the TEL gene on chromosome 12. Transformation of haematopoietic cells occurs through oligomerization of the TEL– PDGFR-β fusion protein, which causes ligand-indepen￾dent constitutive activation of the PDGFR kinase71. Glivec inhibited the growth of cells expressing TEL–PDGFRβ15, and in transgenic mice that expressed the TEL–PDGFRβ, treatment with Glivec inhibited tumour formation and prolonged survival of the ani￾mals72. A remarkable haematological and complete cytogenetic response has been observed in two patients with chronic myeloproliferative disorders associated with a t(5;12) translocation — one of them with a well-characterized TEL–PDGFR fusion gene and the second with a rearranged PDGFR gene with an as yet unidentified partner gene73. Other explor￾atory clinical trials are being carried out in gliomas and in prostate cancer. Targeting the tumour microenvironment An alternative strategy to influence tumour growth is to interfere with the tumour stroma and microvasculature. Paracrine PDGF signalling in the connective-tissue tumour stroma has been described in various types of solid tumour64. Several lines of evidence indicate a role for PDGF in the regulation of interstitial fluid pressure (IFP)74–76. As most solid tumours have an increased IFP, pharmacological reduction might be a way to increase the uptake of anticancer drugs into tumours77. Recent experiments have shown that Glivec significantly reduced tumour IFP in subcutaneously growing PROb rat-colon carcinomas, and a concomitant increase in trans-capillary transport of a radiolabelled tracer com￾pound into the tumour interstitium was observed78. These effects were mediated by inhibition of the express￾ion of PDGFR on blood vessels and stromal cells, as tumour epithelial cells in this tumour model do not express PDGFRs. The angiogenic activity that has been described for PDGF might not only be explained by its direct effects on capillary endothelial cells, pericytes and smooth-muscle