REVIEWS of resistance to alivee in CML Other -KIT-expressingoIn human systemic C of BCR-ABL Mutations in RCR-ARI kin correct binding of Glive Efflux of Glivec (for e mple,by Pep. ciated MDR protein) Protein binding of Glivee (for,tocirculating AGP) BCR-ABL-independent mechanisms(BCR-ABL is inactivated) at n of c-KI Activation of signalling p vs do m of BCR-ABL 8 Activation of leukaemogenic pathways unrelated to BCR-AB ibito arge f 0.1 uM (REE 61).The tor fo m-cell f or(SCF c-KTT,a mer hibition of SC inhibit of MAPK c-Kr with a marked incr e mi in the 5 UM (RE 61的 Development inK-pvGG ted in va us ot hum tumours. it will be imnortant to determine the activ status o crine activatio of thi and acute myeloge. of th c-kIT DO expre utated c ned from the two stru ally sim PDGF-d am to that fou of PDGF cellular e mbryona mic s and inhi tissue.There is increasin his GIST lin ell culture role tum kina d/o asest (REF 57).Two larg g the forr urs by the hu the effect sof tw (400 toohcbeansotnudc URE REVIEWS OLUME 2002 Nature Publishing Group© 2002 Nature Publishing Group NATURE REVIEWS | DRUG DISCOVERY VOLUME 1 | JULY 2002 | 499 REVIEWS Other c-KIT-expressing tumours. In human systemic mastocytosis, most cases show a point mutation in codon 17 of c-KIT, which results in a D816V (aspartic acid 816 to valine) amino-acid substitution in the kinase-2 domain of c-KIT. Interestingly, this mutated c-KIT is resistant to inhibition by Glivec53,58. Expression of c-KIT and SCF has been reported in a retrospective small-cell lung cancer (SCLC) series, indicating that SCLC growth might involve an AUTOCRINE loop. Inhibition of c-KIT activation by transfection of a dominant-negative c-KIT gene results in loss of growth-factor independence59,60. Furthermore, the c-KIT/PDGF-receptor inhibitor AG1296 inhibits growth of SCLC cells in serum-con￾taining medium60. In H526 SCLC cells, pretreatment with Glivec inhibited SCF-mediated c-KIT activation with an IC50 (half-maximal inhibitory concentration) of 0.1 µM (REF. 61). The compound also blocked downstream signal transduction, as evidenced by inhibition of SCF-mediated activation of MAPK and AKT, and potently inhibited SCF-mediated growth in serum-free medium, with a marked increase in apop￾tosis. Glivec also inhibited the growth of SCLC cell lines in a dose-dependent fashion when grown in serum-containing medium; however, the average IC50 was in the range of 5 µM (REFS 61,62). Although c-KIT expression has been documen￾ted in various other human tumours, including acute myelogenous leukaemia, ovarian and testicular cancer, it will be important to determine the activa￾tion status of the receptor and its importance in the pathogenesis (for a review, see REF. 58). Furthermore, it needs to be explored whether pharmacological inhi￾bition of PARACRINE or autocrine activation of this kinase will be successful therapeutically. Exploratory clinical studies are continuing at present in patients with c-KIT-expressing SCLC and acute myeloge￾nous leukaemia. PDGF receptor as a target The third target of Glivec is the PDGF-receptor tyro￾sine kinase. Cellular studies have shown potent inhibi￾tion of the two structurally similar PDGF-α and PDGF-β receptors (PDGFR-α and PDGFR-β), as well as blockade of PDGF-mediated cellular events47,63. PDGF is a connective-tissue-cell mitogen with in vivo functions that include embryonal development, wound healing and control of interstitial-fluid pressure in soft connective tissue. There is increasing evidence that the PDGF ligand–receptor system also has an important role in tumorigenesis64. Paracrine and/or autocrine activation of the PDGFR kinase has been postulated in numerous malignancies, and the pres￾ence of PDGF autocrine loops is most well docu￾mented in gliomas65. Glivec inhibited the in vitro and in vivo growth of cells with autocrine PDGF signalling, including the formation of tumours by the human glioblastoma lines U343 and U87, which had been injected into the brains of nude mice66. The inhibitory effects were mediated predominantly through promo￾tion of growth arrest rather than apoptosis. c-KIT is another target In addition to various oncogenic forms of the BCR–ABL tyrosine kinase, Glivec also inhibits the receptor for stem-cell factor (SCF) — c-KIT, a member of the type III group of receptor kinases. Preclinical studies have established that the drug blocks c-KIT autophosphorylation, as well as SCF-stimulated down￾stream signalling events, such as activation of the mito￾gen-activated protein kinases (MAPKs) ERK1 and ERK2, and AKT (also known as protein kinase B)47,48. Development in c-KIT-positive GISTs. Gastrointestinal stromal tumours (GISTs) represent a rare subset of soft￾tissue sarcomas that involve the gastrointestinal tract and are thought to be derived from the interstitial cells of Cajal. Scientific rationale for the use of Glivec in the treatment of these tumours comes from the landmark work of Hirota et al.49, who first identified somatic gain￾of-function mutations in the c-KIT gene in patients with GIST. Oncogenic c-KIT mutations in GISTs have been localized to the extracellular domain, kinase domains 1 and 2 and predominantly in the juxtamembrane domain of the c-KIT protein50–52.As c-KIT serves as a phenotypic marker of GISTs and has a key role in their pathogenesis, it provides an ideal target for molecular-based therapy. The first evidence that Glivec might inhibit GIST cells that express mutated c-KITwas obtained from studies in a mast-cell leukaemia line expressing a mutated c-KIT similar to that found in GISTs48,53. Furthermore, Glivec rapidly and completely abolished constitutive phospho￾rylation of c-KIT in the human cell line GIST882, which expresses an activating c-KIT mutation in the first part of the cytoplasmic split-tyrosine-kinase domain, and inhib￾ited proliferation in this GIST line54. Similarly, a primary GIST cell culture that expressed a c-KIT exon 11 juxta￾membrane mutation was also inhibited by Glivec54. As reported recently, a pronounced tumour response was first observed in a single patient with progressing GIST55. Following this case report, the high level of effi￾cacy of Glivec in GIST has been shown in two subsequent Phase I (REF. 56) and Phase II studies (REF. 57). Two large Phase III studies are being carried out at present to com￾pare the effectiveness of two doses of Glivec (400 mg or 800 mg daily). On the basis of the Phase II data, the FDA approved the use of Glivec for GISTs on 1 February 2002. AUTOCRINE Describes an agent secreted from a cell that acts on the cell in which it is produced. PARACRINE Describes an agent secreted from a cell that acts on other cells in the local environment. Box 1 | Mechanisms of resistance to Glivec in CML BCR–ABL-dependent mechanisms (cells remain dependent on BCR–ABL signalling) • Amplification of BCR–ABL gene • Mutations in BCR–ABL kinase domain prevent correct binding of Glivec • Efflux of Glivec (for example, by PgP-associated MDR protein) • Protein binding of Glivec (for example, to circulating AGP) BCR–ABL-independent mechanisms (BCR–ABL is inactivated) • Activation of signalling pathways downstream of BCR–ABL • Activation of leukaemogenic pathways unrelated to BCR–ABL AGP,α1-acid glycoprotein; CML, chronic myelogenous leukaemia; MDR, multidrug resistant; PgP, P-glycoprotein