Diverse mechanisms activate the PI 3-kinase/mTOR pathway in melanomas: implications for the use of PI 3-kinase inhibitors to overcome resistance to inhibitors of BRAF and MEK

Background

The PI 3-kinase (PI3K) pathway has been implicated as a target for melanoma therapy.

Methods

Given the high degree of genetic heterogeneity in melanoma, we sought to understand the breadth of variation in PI3K signalling in the large NZM panel of early passage cell lines developed from metastatic melanomas.

Results

We find the vast majority of lines show upregulation of this pathway, and this upregulation is achieved by a wide range of mechanisms. Expression of all class-IA PI3K isoforms was readily detected in these cell lines. A range of genetic changes in different components of the PI3K pathway was seen in different lines. Coding variants or amplification were identified in the PIK3CA gene, and amplification of the PK3CG gene was common. Deletions in the PIK3R1 and PIK3R2 regulatory subunits were also relatively common. Notably, no genetic variants were seen in the PIK3CD gene despite p110δ being expressed in many of the lines. Genetic variants were detected in a number of genes that encode phosphatases regulating the PI3K signalling, with reductions in copy number common in PTEN, INPP4B, INPP5J, PHLLP1 and PHLLP2 genes. While the pan-PI3K inhibitor ZSTK474 attenuated cell growth in all the lines tested, isoform-selective inhibition of p110α and p110δ inhibited cell growth in only a subset of the lines and the inhibition was only partial. This suggests that functional redundancy exists between PI3K isoforms. Furthermore, while ZSTK474 was initially effective in melanoma cells with induced resistance to vemurafenib, a subset of these cell lines concurrently developed partial resistance to PI3K inhibition. Importantly, mTOR-selective or mTOR/PI3K dual inhibitors effectively inhibited cell growth in all the lines, including those already resistant to BRAF inhibitors and ZSTK474.

Conclusions

Overall, this indicates a high degree of diversity in the way the PI3K pathway is activated in different melanoma cell lines and that mTOR is the most effective point for targeting the growth via the PI3K pathway across all of these cell lines.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-07826-4.

Potentiation of Growth Inhibitory Responses of the mTOR Inhibitor Everolimus by Dual mTORC1/2 Inhibitors in Cultured Breast Cancer Cell Lines

The mammalian target of rapamycin (mTOR), a vital component of signaling pathways involving PI3K/AKT, is an attractive therapeutic target in breast cancer. Everolimus, an allosteric mTOR inhibitor that inhibits the mTOR functional complex mTORC1, is approved for treatment of estrogen receptor positive (ER+) breast cancer. Other mTOR inhibitors show interesting differences in target specificities: BEZ235 and GSK2126458 are ATP competitive mTOR inhibitors targeting both PI3K and mTORC1/2; AZD8055, AZD2014 and KU-0063794 are ATP competitive mTOR inhibitors targeting both mTORC1 and mTORC2; and GDC-0941 is a pan-PI3K inhibitor. We have addressed the question of whether mTOR inhibitors may be more effective in combination than singly in inhibiting the proliferation of breast cancer cells. We selected a panel of 30 human breast cancer cell lines that included ER and PR positive, HER2 over-expressing, and “triple negative” variants, and determined whether signaling pathway utilization was related to drug-induced inhibition of proliferation. A significant correlation (p = 0.005) was found between everolimus IC₅₀ values and p70S6K phosphorylation, but not with AKT or ERK phosphorylation, consistent with the mTOR pathway being a principal target. We then carried out combination studies with four everolimus resistant triple-negative breast cancer cell lines, and found an unexpectedly high degree of synergy between everolimus and the other inhibitors tested. The level of potentiation of everolimus inhibitory activity (measured by IC₅₀ values) was found to be cell line-specific for all the kinase inhibitors tested. The results suggest that judicious combination of mTOR inhibitors with different modes of action could have beneficial effects in the treatment of breast cancer.

Inhibitors of pan-PI3K Signaling Synergize with BRAF or MEK Inhibitors to Prevent BRAF-Mutant Melanoma Cell Growth

BRAF and MEK inhibitors have improved outcomes for patients with BRAF-mutant melanoma, but their efficacy is limited by both intrinsic and acquired resistances. Activation of the PI3K pathway can mediate resistance to these agents, providing a strong rationale for combination therapy in melanoma. Here, a panel of nine low-passage human metastatic melanoma cell lines with BRAF mutations was tested in cell proliferation and protein expression assays for sensitivity to inhibitors of MEK (selumetinib) and BRAF (vemurafenib) as single agents and in combination with inhibitors of pan-PI3K (ZSTK474), pan-PI3K/mTOR (BEZ235), individual PI3K isoforms (p110α, A66; p110β, TGX-221; p110γ, AS-252424; p110δ, idelalisib), or mTORC1/2 (KU-0063794). Selumetinib and vemurafenib potently inhibited cell proliferation in all cell lines, especially in those that expressed low levels of phosphorylated AKT (pAKT). ZSTK474 and BEZ235 also inhibited cell proliferation in all cell lines and enhanced the antitumor activity of selumetinib and vemurafenib in the majority of lines by either interacting synergistically or additively to increase potency or by inducing cytotoxicity by significantly increasing the magnitude of cell growth inhibition. Furthermore, ZSTK474 or BEZ235 combined with selumetinib to produce robust inhibition of pERK, pAKT, and pS6 expression and synergistic inhibition of NZM20 tumor growth. The inhibitors of individual PI3K isoforms or mTORC1/2 were less effective at inhibiting cell proliferation either as single agents or in combination with selumetinib or vemurafenib, although KU-0063794 synergistically interacted with vemurafenib and increased the magnitude of cell growth inhibition with selumetinib or vemurafenib in certain cell lines. Overall, these results suggest that the sensitivity of BRAF-mutant melanoma cells to BRAF or MEK inhibitors is at least partly mediated by activation of the PI3K pathway and can be enhanced by combined inhibition of the BRAF/MEK and PI3K/mTOR signaling pathways.

Abstract 1644: Design and discovery of PWT33597 (VDC-597), a dual inhibitor of PI3-kinase alpha and mTOR

Phosphoinositide-3-kinase (PI3K) is an important mediator of tumor cell growth, survival and proliferation. In particular, PI3K alpha is important for signaling downstream of receptor tyrosine kinases and is also frequently amplified or mutationally

activated in tumors, suggesting that selective inhibitors of this isoform may have therapeutic utility in the treatment of cancer. Downstream of PI3K, the mTOR kinase also plays a critical role in cellular growth and metabolism, and inhibitors of mTOR have demonstrated clinical benefit in several tumor types. We report here the design, discovery and characterization of PWT33597 (VDC-597), a dual inhibitor of PI3K alpha and mTOR, which entered human clinical trials in 2011. Starting with the known pan-Class I PI3-kinase inhibitor ZSTK474, we identified the methanesulfonylpiperazine analogue, 2-(difluoromethyl)-1-[4-[4-(methylsulfonyl)-1-piperazinyl]-6-(4-morpholinyl)-1,3,5-triazin-2-yl]-1H-benzimidazole as a promising lead compound with activity against both PI3K alpha (IC50 = 21 nM) and PI3K delta (IC50 = 18 nM). The addition of a methoxy group at the 4-position of the benzimidazole group led to a more selective inhibitor of PI3K alpha (IC50 = 6 nM versus 41 nM for PI3K delta), although with reduced solubility. A search for more soluble analogues identified SN 32976 as a selective inhibitor of PI3K alpha (IC50 = 28 nM) over both PI3K delta (IC50 = 287 nM) and mTOR (IC50 = 227 nM), with good aqueous solubility. SN 32976 displayed good oral bioavailability and was significantly more active than ZSTK474 against a U87 MG human tumor xenograft model in mice. A search for more metabolically stable analogues subsequently identified PWT33597, which maintained the selectivity for PI3K alpha (IC50 = 26 nM) over PI3K delta (IC50 = 291 nM) but now also displayed activity against mTOR in biochemical assays (IC50 = 21 nM). PWT33597 had good pharmacokinetic properties in multiple preclinical species, was not extensively metabolized in vivo and showed little potential for interaction with cytochrome P450 enzymes. Human clinical trials of PWT33597 were completed in 2012, and it is now undergoing further studies in veterinary cancers (as VDC-597).

Citation Format: Gordon W. Rewcastle, Jack U. Flanagan, Anna C. Giddens, Swarna A. Gamage, Sophia KY Tsang, Jackie D. Kendall, Bruce C. Baguley, Christina M. Buchanan, David J. Matthews, Marie O'Farrell, Stephen MF Jamieson, William A. Denny, Peter R. Shepherd. Design and discovery of PWT33597 (VDC-597), a dual inhibitor of PI3-kinase alpha and mTOR. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1644. doi:10.1158/1538-7445.AM2014-1644

Relationships between signaling pathway usage and sensitivity to a pathway inhibitor: examination of trametinib responses in cultured breast cancer lines

Cellular signaling pathways involving mTOR, PI3K and ERK have dominated recent studies of breast cancer biology, and inhibitors of these pathways have formed a focus of numerous clinical trials. We have chosen trametinib, a drug targeting MEK in the ERK pathway, to address two questions. Firstly, does inhibition of a signaling pathway, as measured by protein phosphorylation, predict the antiproliferative activity of trametinib? Secondly, do inhibitors of the mTOR and PI3K pathways synergize with trametinib in their effects on cell proliferation? A panel of 30 human breast cancer cell lines was chosen to include lines that could be classified according to whether they were ER and PR positive, HER2 over-expressing, and “triple negative”. Everolimus (targeting mTOR), NVP-BEZ235 and GSK2126458 (both targeting PI3K/mTOR) were chosen for combination experiments. Inhibition of cell proliferation was measured by IC₅₀ values and pathway utilization was measured by phosphorylation of signaling kinases. Overall, no correlation was found between trametinib IC₅₀ values and inhibition of ERK signaling. Inhibition of ERK phosphorylation was observed at trametinib concentrations not affecting proliferation, and sensitivity of cell proliferation to trametinib was found in cell lines with low ERK phosphorylation. Evidence was found for synergy between trametinib and either everolimus, NVP-BEZ235 or GSK2126458, but this was cell line specific. The results have implications for the clinical application of PI3K/mTOR and MEK inhibitors.

Extended treatment with selective phosphatidylinositol 3-kinase and mTOR inhibitors has effects on metabolism, growth, behaviour and bone strength

The class I phosphatidylinositol 3-kinases (PtdIns3Ks) mediate the effects of many hormones and growth factors on a wide range of cellular processes, and activating mutations or gene amplifications of class I PtdIns3K isoforms are known to contribute to oncogenic processes in a range of tumours. Consequently, a number of small-molecule PtdIns3K inhibitors are under development and in clinical trial. The central signalling role of PtdIns3K in many cellular processes suggests there will be on-target side effects associated with the use of these agents. To gain insights into what these might be we investigated the effect of extended daily dosing of eight small-molecule inhibitors of class Ia PtdIns3Ks. Animals were characterized in metabolic cages to analyse food intake, oxygen consumption and movement. Insulin tolerance and body composition were analysed at the end of the experiment, the latter using EchoMRI. Bone volume and strength was assessed by micro-CT and three-point bending, respectively. Surprisingly, after sustained dosing with pan-PtdIns3K inhibitors and selective inhibitors of the p110α isoform there was a resolution of the impairments in insulin tolerance observed in drug-naïve animals treated with the same drugs. However, pan-PtdIns3K inhibitors and selective inhibitors of the p110α have deleterious effects on animal growth, animal behaviour and bone volume and strength. Together, these findings identify a range of on target effects of PtdIns3K inhibitors and suggest use of these drugs in humans may have important adverse effects on metabolism, body composition, behaviour and skeletal health.

Abstract B162: Inhibition of the p110α

Signaling through the phosphoinositide 3-kinase (PI3K) pathway promotes cell survival and proliferation, but is also involved in the regulation of glucose metabolism. The PI3K pathway is frequently deregulated in cancer through amplification or mutational activation of the PIK3CA oncogene or deletion of the PTEN tumor suppressor gene. Therefore, PI3K has become a promising target for anticancer drug development, with several drugs that target multiple PI3K isoforms and mTOR (mammalian target of rapamycin) currently under clinical development. However, although class Ia PI3K oncogenic mutants have only been identified for the PIK3CA-encoded p110α isoform, there is a lack of drugs that have been developed to specifically target p110α and to identify the impact of p110α inhibition on tumor growth and glucose metabolism. In this study, we have investigated small molecule inhibitors of p110α that specifically target cells harboring the PIK3CA H1047R mutation. The selectivity of the inhibitors was confirmed by their potency against different PI3K isoforms and cellular activity was determined by their effect on pAkt expression. Tumor xenograft models with PIK3CA, PIK3CA/KRAS or PTEN mutations were used to evaluate the effect of p110α inhibition on tumor growth, while the role of p110α in glucose metabolism was investigated by tolerance to insulin, pyruvate and glucose in mice treated with PI3K inhibitors. Inhibition of p110α alone knocked down pAkt expression, with the greatest effect observed in cell lines that harbored the PIK3CA H1047R mutation and were wild-type for KRAS. A similar pattern emerged in human tumor xenograft models, where inhibition of p110 resulted in greater antitumor efficacy in SK-OV-3 (PIK3CA H1047R mutation) tumors than in HCT-116 (PIK3CA H1047R, KRAS G13D) or U87MG (PTEN deletion) tumors. In addition to delaying tumor growth, p110α inhibition also resulted in impaired glucose metabolism. The blood glucose or plasma insulin levels after administration of insulin, pyruvate or glucose were significantly elevated for up to 3 hrs in mice treated with p110α inhibitors relative to blood glucose or plasma insulin levels in control-treated mice. Similar effects on glucose metabolism were observed for pan-PI3K inhibitors, but not for selective inhibitors of other PI3K class Ia isoforms: p110α or p110α. These results suggest that selective targeting of the p110α isoform of PI3K is a viable anticancer therapeutic option, particularly for the subset of cancers harboring PIK3CA mutations. However, since p110α also appears to be necessary for regular glucose metabolism in vivo, treatment with p110α-selective PI3K inhibitors is likely to be adversely associated with hyperglycemia.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B162.

Synthesis and biological evaluation of novel analogues of the pan class I phosphatidylinositol 3-kinase (PI3K) inhibitor 2-(difluoromethyl)-1-[4,6-di(4-morpholinyl)-1,3,5-triazin-2-yl]-1H-benzimidazole (ZSTK474)

A structure-activity relationship (SAR) study of the pan class I PI 3-kinase inhibitor 2-(difluoromethyl)-1-[4,6-di(4-morpholinyl)-1,3,5-triazin-2-yl]-1H-benzimidazole (ZSTK474) identified substitution at the 4 and 6 positions of the benzimidazole ring as having significant effects on the potency of substituted derivatives. The 6-amino-4-methoxy analogue displayed a greater than 1000-fold potency enhancement over the corresponding 6-aza-4-methoxy analogue against all three class Ia PI 3-kinase enzymes (p110α, p110β, and p110δ) and also displayed significant potency against two mutant forms of the p110α isoform (H1047R and E545K). This compound was also evaluated in vivo against a U87MG human glioblastoma tumor xenograft model in Rag1(-/-) mice, and at a dose of 50 mg/kg given by ip injection at a qd × 10 dosing schedule it dramatically reduced cancer growth by 81% compared to untreated controls.

Comparison of the effects of the PI3K/mTOR inhibitors NVP-BEZ235 and GSK2126458 on tamoxifen-resistant breast cancer cells

BACKGROUND

Treatment with anti-estrogens or aromatase inhibitors is commonly used for patients with estrogen receptor-positive (ER+) breast cancers; however resistant disease develops almost inevitably, requiring a choice of secondary therapy. One possibility is to use inhibitors of the PI3K/mTOR pathway and several candidate drugs are in development. We examined the in vitro effects of two inhibitors of the PI3K/mTOR pathway on resistant MCF-7 cells.

METHODS

We cultured MCF-7 cells for prolonged periods either in the presence of the anti-estrogen tamoxifen (3 sub-lines) or in estrogen free medium (2 sub-lines) to mimic the effects of clinical treatment. We then analyzed the effects of two dual PI3K/mTOR phosphoinositide-3-kinase inhibitors, NVP-BEZ235 and GSK2126458, on the growth and signaling pathways of these MCF-7 sub-lines. The functional status of the PI3K, mTOR and ERK pathways was analyzed by measuring phosphorylation of AKT, p70S6K, rpS6 and ERK.

RESULTS

The derived sub-lines showed increased resistance to tamoxifen but none exhibited concomitantly increased sensitivity to the PI3K inhibitors. NVP-BEZ235 and GSK2126458 acted mainly by induction of cell cycle arrest, particularly in G1-phase, rather than by induction of apoptosis. The lines varied considerably in their utilization of the AKT, p70S6K and ERK pathways. NVP-BEZ235 and GSK2126458 inhibited AKT signaling but NVP-BEZ235 showed greater effects than GSK2126458 on p70S6K and rpS6 signaling with effects resembling those of rapamycin.

CONCLUSION

Increased resistance to tamoxifen in these MCF-7 sub-lines is not associated with hypersensitivity to PI3K inhibitors. While both drugs inhibited AKT signaling, NVP-BEZ235 resembled rapamycin in inhibiting the mTOR pathway.

4-Anilino-5-carboxamido-2-pyridone Derivatives as Noncompetitive Inhibitors of Mitogen-Activated Protein Kinase Kinase

A new series of MEK1 inhibitors, the 4-anilino-5-carboxamido-2-pyridones, were designed and synthesized using a combination of medicinal chemistry, computational chemistry, and structural elucidation. The effect of variation in the carboxamide side chain, substitution on the pyridone nitrogen, and replacement of the 4'-iodide were all investigated. This study afforded several compounds which were either equipotent or more potent than the clinical candidate CI-1040 (1) in an isolated enzyme assay, as well as murine colon carcinoma (C26) cells, as measured by suppression of phosphorylated ERK substrate. Most notably, pyridone 27 was found to be more potent than 1 in vitro and produced a 100% response rate at a lower dose than 1, when tested for in vivo efficacy in animals bearing C26 tumors.

Synthesis, biological evaluation and molecular modelling of sulfonohydrazides as selective PI3K p110alpha inhibitors

A series of 2-methyl-5-nitrobenzenesulfonohydrazides were prepared and evaluated as inhibitors of PI3K. An isoquinoline derivative shows good selectivity for the p110alpha isoform over p110beta and p110delta, and also demonstrates good in vitro activity in a cell proliferation assay. Molecular modelling provides a rationalisation for the observed SAR.

Identification of a unique co-operative phosphoinositide 3-kinase signaling mechanism regulating integrin alpha IIb beta 3 adhesive function in platelets

Phosphoinositide (PI) 3-kinases play an important role in regulating the adhesive function of a variety of cell types through affinity modulation of integrins. Two type I PI 3-kinase isoforms (p110 beta and p110 gamma) have been implicated in G(i)-dependent integrin alpha(IIb)beta(3) regulation in platelets, however, the mechanisms by which they coordinate their signaling function remains unknown. By employing isoform-selective PI 3-kinase inhibitors and knock-out mouse models we have identified a unique mechanism of PI 3-kinase signaling co-operativity in platelets. We demonstrate that p110 beta is primarily responsible for G(i)-dependent phosphatidylinositol 3,4-bisphosphate (PI(3,4)P(2)) production in ADP-stimulated platelets and is linked to the activation of Rap1b and AKT. In contrast, defective integrin alpha(IIb)beta(3) activation in p110 gamma(-/-) platelets was not associated with alterations in the levels of PI(3,4)P(2) or active Rap1b/AKT. Analysis of the effects of active site pharmacological inhibitors confirmed that p110 gamma principally regulated integrin alpha(IIb)beta(3) activation through a non-catalytic signaling mechanism. Inhibition of the kinase function of PI 3-kinases, combined with deletion of p110 gamma, led to a major reduction in integrin alpha(IIb)beta(3) activation, resulting in a profound defect in platelet aggregation, hemostatic plug formation, and arterial thrombosis. These studies demonstrate a kinase-independent signaling function for p110 gamma in platelets. Moreover, they demonstrate that the combined catalytic and non-catalytic signaling function of p110 beta and p110 gamma is critical for P2Y(12)/G(i)-dependent integrin alpha(IIb)beta(3) regulation. These findings have potentially important implications for the rationale design of novel antiplatelet therapies targeting PI 3-kinase signaling pathways.

Phenazine-1-carboxamides: Structure–cytotoxicity relationships for 9-substituents and changes in the H-bonding pattern of the cationic side chain

A series of phenazine-1-carboxamides were prepared, including variations in both chromophore substituents and the nature of the cationic side chain. The novel side-chain analogues were prepared from the corresponding phenazine-1-carboxylic acids via Schmidt conversion to the 1-amines and from the corresponding 1-halides. Structure-cytotoxicity relationships for these compounds in a panel of tumor cell lines showed that there is very limited scope for variation of the structure of the 1-carboxamide side chain, consistent with the recent structural model of how tricyclic carboxamides bind to DNA. There was generally little difference in IC(50)s between parent and P-glycoprotein expressing cell lines, suggesting that most of the compounds are not affected by the presence of this efflux pump.

Becatecarin (Helsinn Healthcare)

Helsinn Healthcare SA is developing becatecarin, a topoisomerase II inhibitor for the potential intravenous treatment of cancer.

Palladium-Catalyzed Cross-Coupling in the Synthesis of Pyridinyl Boxazomycin C Analogues

[reaction: see text] Palladium-catalyzed cross-coupling of 2-thiomethylbenzoxazoles with tri-alkylstannyl pyridines efficiently produces pyridinyl boxazomycin C analogues.

Structure–activity relationships for 2-anilino-6-phenylpyrido[2,3-d]pyrimidin-7(8H)-ones as inhibitors of the cellular checkpoint kinase Wee1

A series of 2-anilino-6-phenylpyrido[2,3-d]pyrimidin-7(8H)-ones were synthesized and evaluated for their inhibitory properties against the non-receptor kinase c-Src and the G2/M checkpoint kinase Wee1. Overall, the compounds were 10-100-fold more potent inhibitors of c-Src than Wee1, and variation of substituents on the 6-phenyl ring did not markedly alter this preference. Solubilizing substituents off the 2-anilino ring in many cases increased Wee1 activity, thus lowering this preference to about 10-fold. 5-Alkyl substituted analogs were generally Wee1 selective, but at the expense of absolute potency.

Potential antitumor agents. 41. Analogs of amsacrine with electron-donor substituents in the anilino ring

The preparation and antitumor activity of a series of 3'-alkylamino and 3'-dialkylamino analogues of amsacrine are reported. The results support previous work suggesting that the presence of electron-donating groups in the 3'-position of the anilino ring substantially enhance the antitumor activity of amsacrine analogues, possibly by the provision of high levels of electron density at the 6'-position. The alkylamino derivatives generally possess tighter DNA binding, higher levels of in vitro and in vivo antileukemic activity, and greater aqueous solubility than the corresponding amsacrine analogues.

Structure-activity relationships for pyrido-, imidazo-, pyrazolo-, pyrazino-, and pyrrolophenazinecarboxamides as topoisomerase-targeted anticancer agents

Heterocyclic phenazinecarboxamides were prepared by condensation of aminoheterocycles and 2-halo-3-nitrobenzoic acids, followed by reductive ring closure and amidation. They showed similar inhibition of paired cell lines that underexpressed topo II or overexpressed P-glycoprotein, indicating a non topo II mechanism of cytotoxicity and indifference to P-glycoprotein mediated multidrug resistance. Compounds with a fused five-membered heterocyclic ring were generally less potent than the pyrido[4,3-a]phenazines. A 4-methoxypyrido[4,3-a]phenazine (IC(50)s 2.5-26 nM) gave modest (ca. 5 day) growth delays in H69/P xenografts with oral dosing.

Structure-activity relationships for pyrido-, imidazo-, pyrazolo-, pyrazino-, and pyrrolophenazinecarboxamides as topoisomerase-targeted anticancer agents

Heterocyclic phenazinecarboxamides were prepared by condensation of aminoheterocycles and 2-halo-3-nitrobenzoic acids, followed by reductive ring closure and amidation. They showed similar inhibition of paired cell lines that underexpressed topo II or overexpressed P-glycoprotein, indicating a non topo II mechanism of cytotoxicity and indifference to P-glycoprotein mediated multidrug resistance. Compounds with a fused five-membered heterocyclic ring were generally less potent than the pyrido[4,3-a]phenazines. A 4-methoxypyrido[4,3-a]phenazine (IC(50)s 2.5-26 nM) gave modest (ca. 5 day) growth delays in H69/P xenografts with oral dosing.

Synthesis and structure-activity relationships of 7-substituted 3-(2, 6-dichlorophenyl)-1,6-naphthyridin-2(1H)-ones as selective inhibitors of pp60(c-src)

7-substituted 3-(2,6-dichlorophenyl)-1,6-naphthyridin-2(1H)-ones are potent inhibitors of protein tyrosine kinases, with some selectivity for c-Src. The compounds were prepared by condensing 4, 6-diaminonicotinaldehyde with 2,6-dichlorophenylacetonitrile and selectively converting the 2- and 7-amino groups of the product to hydroxy and fluoro groups, respectively, by prolonged diazotization in 50% aqueous fluoboric acid. N-Methylation, followed by treatment with aliphatic diamines, aromatic amines, or their derived lithium anions, gave the desired compounds. Selected isomeric 1, 8-naphthyridin-2(1H)-ones were also prepared in order to evaluate the relative contributions of both ring A aza atoms of the related pyrido[2,3-d]pyrimidin-7(8H)-ones to the inhibitory activity. The compounds were evaluated for their ability to prevent phosphorylation of a model substrate by c-Src, FGF-1 receptor, and PDGF-beta receptor enzymes. Overall, there was a high degree of correlation of the activities against the different kinases, with c-Src being generally the most sensitive to structural changes. 1, 6-Naphthyridin-2(1H)-one analogues bearing basic aliphatic side chains [7-NH(CH(2))(n)()NRR, 7-NHPhO(CH(2))(n)()NRR, or 7-NHPhN(CH(2))(4)NMe] were the most potent against c-Src (IC(50)s of 10-80 nM), showing good selectivity with respect to PDGFR (10-300-fold) but less with respect to FGFR. The 1, 6-naphthyridin-2(1H)-ones showed broadly similar activity to the analogous pyrido[2,3-d]pyrimidin-7(8H)-ones, whereas the 1, 8-naphthyridin-2(1H)-ones were at least 10(3)-fold less potent. These results, indicating that the 3-aza atom in the pyrido[2, 3-d]pyrimidin-7(8H)-ones is mandatory, whereas the 1-aza atom is not, support the published binding model for these compounds to c-Src (J. Med. Chem. 1998, 41, 1752), where the 3-aza and 2-NH atoms form a bidentate H-bond donor-acceptor motif that interacts with Met341 and the 1-aza atom is not involved in specific binding interactions.

Bis(phenazine-1-carboxamides): structure-activity relationships for a new class of dual topoisomerase I/II-directed anticancer drugs

Ring-substituted bis(phenazine-1-carboxamides), linked by a -(CH(2))(3)NMe(CH(2))(3)- chain, were prepared from the corresponding substituted phenazine-1-carboxylic acids by reaction of the intermediate imidazolides with bis(3-aminopropyl)methylamine. The compounds were evaluated for growth inhibitory activity in a panel of tumor cell lines, including P388 leukemia, Lewis lung carcinoma, and wild-type (JL(C)) and mutant (JL(A) and JL(D)) forms of human Jurkat leukemia. The latter mutant lines are resistant to topoisomerase (topo) II targeted agents because of lower levels of the enzyme. Analogues with small, lipophilic substituents (e.g., Me, Cl) at the 9-position were the most potent inhibitors, superior to the corresponding dimeric bis(acridine-4-carboxamides) (bis-DACA analogues). Several of the compounds were preferentially (up to 2-fold) more cytotoxic toward the mutant Jurkat lines than the wild-type. To test whether this selectivity was related to topoisomerase action, the most potent of the compounds (9-methyl) was evaluated in a cell-free system. It poisoned topo I at drug concentrations of 0.25 and 0.5 microM and inhibited the catalytic activity of both topo I and topo II at concentrations of 1 and 5 microM, respectively. Results from the NCI human tumor cell line panel showed the compounds had preferential activity toward colon tumor lines (on average 9.5-fold more active in the HT29 line than in the cell line panel as a whole). Several analogues produced significant growth delays in the relatively refractory subcutaneous colon 38 tumor model in vivo. In particular, the 9-methyl compound was substantially more potent in this tumor model than the clinical dual topo I/II poison DACA (total dose 90 versus 400 mg/kg) with comparable activity. The bis(phenazine-1-carboxamides) are a new and interesting class of dual topo I/II-directed anticancer drugs.

Tyrosine kinase inhibitors. 17. Irreversible inhibitors of the epidermal growth factor receptor: 4-(phenylamino)quinazoline- and 4-(phenylamino)pyrido[3,2-d]pyrimidine-6-acrylamides bearing additional solubilizing functions

4-Anilinoquinazoline- and 4-anilinopyrido[3,2-d]pyrimidine-6-acrylamides substituted with solubilizing 7-alkylamine or 7-alkoxyamine side chains were prepared by reaction of the corresponding 6-amines with acrylic acid or acrylic acid anhydrides. In the pyrido[3,2-d]pyrimidine series, the intermediate 6-amino-7-alkylamines were prepared from 7-bromo-6-fluoropyrido[3,2-d]pyrimidine via Stille coupling with the appropriate stannane under palladium(0) catalysis. This proved a versatile method for the introduction of cationic solubilizing side chains. The compounds were evaluated for their inhibition of phosphorylation of the isolated EGFR enzyme and for inhibition of EGF-stimulated autophosphorylation of EGFR in A431 cells and of heregulin-stimulated autophosphorylation of erbB2 in MDA-MB 453 cells. Quinazoline analogues with 7-alkoxyamine solubilizing groups were potent irreversible inhibitors of the isolated EGFR enzyme, with IC(50[app]) values from 2 to 4 nM, and potently inhibited both EGFR and erbB2 autophosphorylation in cells. 7-Alkylamino- and 7-alkoxyaminopyrido[3,2-d]pyrimidines were also irreversible inhibitors with equal or superior potency against the isolated enzyme but were less effective in the cellular autophosphorylation assays. Both quinazoline- and pyrido[3,2-d]pyrimidine-6-acrylamides bound at the ATP site alkylating cysteine 773, as shown by electrospray ionization mass spectrometry, and had similar rates of absorptive and secretory transport in Caco-2 cells. A comparison of two 7-propoxymorpholide analogues showed that the pyrido[3,2-d]pyrimidine-6-acrylamide had greater amide instability and higher acrylamide reactivity, being converted to glutathione adducts in cells more rapidly than the corresponding quinazoline. This difference may contribute to the observed lower cellular potency of the pyrido[3,2-d]pyrimidine-6-acrylamides. Selected compounds showed high in vivo activity against A431 xenografts on oral dosing, with the quinazolines being superior to the pyrido[3,2-d]pyrimidines. Overall, the quinazolines proved superior to previous analogues in terms of aqueous solubility, potency, and in vivo antitumor activity, and one example (CI 1033) has been selected for clinical evaluation.

Tyrosine kinase inhibitors. 15. 4-(Phenylamino)quinazoline and 4-(phenylamino)pyrido[d]pyrimidine acrylamides as irreversible inhibitors of the ATP binding site of the epidermal growth factor receptor

A series of 6- and 7-acrylamide derivatives of the 4-(phenylamino)quinazoline and -pyridopyrimidine classes of epidermal growth factor receptor (EGFR) inhibitors were prepared from the corresponding amino compounds by reaction with either acryloyl chloride/base or acrylic acid/1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride. All of the 6-acrylamides, but only the parent quinazoline 7-acrylamide, were irreversible inhibitors of the isolated enzyme, confirming that the former are better-positioned, when bound to the enzyme, to react with the critical cysteine-773. Quinazoline, pyrido[3,4-d]pyrimidine, and pyrido[3,2-d]pyrimidine 6-acrylamides were all irreversible inhibitors and showed similar high potencies in the enzyme assay (likely due to titration of the available enzyme). However the pyrido[3,2-d]pyrimidine analogues were 2-6-fold less potent than the others in a cellular autophosphorylation assay for EGFR in A431 cells. The quinazolines were generally less potent overall toward inhibition of heregulin-stimulated autophosphorylation of erbB2 (in MDA-MB-453-cells), whereas the pyridopyrimidines were equipotent. Selected compounds were evaluated in A431 epidermoid and H125 non-small-cell lung cancer human tumor xenografts. The compounds showed better activity when given orally than intraperitoneally. All showed significant tumor growth inhibition (stasis) over a dose range. The poor aqueous solubility of the compounds was a drawback, requiring formulation as fine particulate emulsions.

Plasma disposition, metabolism and excretion of the experimental antitumour agent 5,6-dimethylxanthenone-4-acetic acid in the mouse, rat and rabbit

5,6-Dimethylxanthenone-4-acetic acid (DMXAA), an experimental antitumour agent currently undergoing phase I clinical trial, has a maximum tolerated dose (MTD) in male BDF1 mice of 99 micromol/kg. We have found the male Sprague-Dawley rat and the New Zealand White rabbit to have greater tolerance to DMXAA, with MTDs being 990 and 330 micromol/kg, respectively. To investigate the causes of this difference, we measured plasma and urine DMXAA concentrations by high-performance liquid chromatography (HPLC) after single i.v. bolus injections of 99 and 990 micromol/kg in the rat and following a bolus dose of 99 micromol/kg and a 10-min infusion of 330 micromol/kg in the rabbit. Following administration of DMXAA at the MTD in the mouse, rat and rabbit the maximal concentrations were 600, 2,200 and 1,708 microM, respectively, whereas areas under the concentration-time curves were 2,400, 19,000 and 2,400 microMh, respectively, for unchanged DMXAA. Data obtained for mice and rabbits were satisfactorily fitted to a two-compartment model with Michaelis-Menten kinetics. DMXAA was highly bound to plasma proteins, with the highest degree of binding being found in the rabbit. A small proportion of the total dose (7.8%, 0.6% and 12.4%, respectively) was excreted unchanged in urine over 24 h. This proportion increased (to 11.6%, 3.5% and 72.4%, respectively) following alkaline hydrolysis, suggesting the presence of glucuronide metabolites. Examination of rat and mouse urine by HPLC revealed the presence of two metabolites, which were characterized by mass spectrometry and nuclear magnetic resonance to be the acyl glucuronide of DMXAA and 6-(hydroxymethyl)-5-methylxanthenone-4-acetic acid. Thus, both mice and rats metabolise DMXAA by similar pathways. The results demonstrate considerable interspecies variations in tolerance to DMXAA that cannot be explained by differences in pharmacokinetics.

Inhibition of growth of primary human tumour cell cultures by a 4-anilinoquinazoline inhibitor of the epidermal growth factor receptor family of tyrosine kinases

The epidermal growth factor receptor (EGFR) is thought to mediate the action of the mitogens EGF and tumour growth factor-alpha (TGF-alpha) in a variety of cancers, including those of the lung, breast and ovary. A number of new selective inhibitors of EGFR tyrosine kinase have now been developed as potential new antitumour agents. We used a potent inhibitor of this tyrosine kinase, 6-amino-4-[(3-bromophenyl)amino]-7-(methylamino)quinazoline (SN 25531; PD 156273), to determine the responses of primary cultures derived from patients with cancer of the lung, ovary, breast, cervix and endometrium. Cells were cultured in 96-well plates and proliferation assessed by incorporation of 3H-thymidine. Measured growth inhibitory concentrations IC50 values) varied from 1 nM to 14 microM with a 1000-fold differential between sensitive and resistant cultures. Results were compared with rates of proliferation, estimated using a paclitaxel-based method. We also measured the IC50 values for the tyrosine kinase inhibitor using a number of established human cell lines, and compared them with EGFR content using fluorescent antibody staining and flow cytometry. The presence of EGFR was found to be necessary, but not sufficient, for in vitro response. Only a small number of cell lines (3 of 7 for lung, 1 of 7 for ovarian, 2 of 3 squamous cell and 0 of 12 for melanoma) were sensitive to the tyrosine kinase inhibitor. In contrast, 40 of the 50 primary cultures (including 14 of 15 lung cancer samples and 14 of 19 ovarian cancer samples) were sensitive.

Tyrosine kinase inhibitors. 14. Structure-activity relationships for methylamino-substituted derivatives of 4-[(3-bromophenyl)amino]-6-(methylamino)-pyrido[3,4-d]pyrimidine (PD 158780), a potent and specific inhibitor of the tyrosine kinase activity of receptors for the EGF family of growth factors

The 4-[(3-bromophenyl)amino]pyrido[3,4-d]pyrimidine PD 158780 is a very potent in vitro inhibitor of the tyrosine kinase activity of the epidermal growth factor receptor (EGFR) (IC50 0.08 nM), and other members of the erbB family, by competitive binding at the ATP site of these signal transduction enzymes. A series of analogues of PD 158780 bearing solubilizing functions off the 6-methylamino substituent were prepared by reaction of the 6-fluoro derivatives with appropriate amine nucleophiles. These were evaluated for their ability to inhibit the tyrosine phosphorylating action of EGF-stimulated full-length EGFR enzyme and for inhibition of autophosphorylation of the EGFR in A431 human epidermoid carcinoma cells in culture. The most effective analogues were those bearing weakly basic substituents through a secondary amine linkage, which proved water-soluble (> 10 mM) and potent (IC50S generally < 1 nM). No clear SAR could be discerned for these compounds with respect to amine base strength or the distance of the cationic center from the chromophore, suggesting that 6-substituents are in a favorable area of bulk tolerance in the enzyme binding site. More distinct SAR emerged for the ability of the compounds to inhibit EGFR autophosphorylation in A431 cells, where analogues bearing lipophilic weak bases were preferred. Representative analogues were evaluated for antitumor effectiveness against four in vivo tumor models. Significant in vivo activity was observed in estrogen-dependent MCF-7 breast and A431 epidermoid tumors. Marginal activity was seen in an EGFR-transfected tumor model, suggesting that while this cell line requires EGF for clone formation in soft agar, other growth factors may be able to replace EGF in vivo. Also, no activity was seen against the SK-OV-3 ovarian cancer model, which is known to express other EGF receptor family members (although it is not clear whether these are absolutely required for growth in vivo). While substantial growth delays were seen in A431 and MCF-7 tumor models, the treated tumors remained approximately the same size throughout therapy, suggesting that the compounds are cytostatic rather than cytotoxic under these test conditions. It remains to be determined if more prolonged therapy has cytotoxic effects in vivo, resulting in net tumor cell kill.

Biochemical and antiproliferative properties of 4-[ar(alk)ylamino]pyridopyrimidines, a new chemical class of potent and specific epidermal growth factor receptor tyrosine kinase inhibitor

The tyrosine kinase inhibitors PD 69896, 153717, and 158780, which belong to the chemical class 4-[ar(alk)ylamino]pyridopyrimidines, have been characterized with respect to enzymology, target specificity, and antiproliferative effects in tumor cells. These compounds were competitive inhibitors with respect to ATP against purified epidermal growth factor (EGF) receptor tyrosine kinase and inhibited EGF receptor autophosphorylation in A431 human epidermoid carcinoma with IC50 values of 2085, 110, and 13 nM, respectively. Onset of inhibition was immediate once cells were exposed to these compounds, whereas recovery of receptor autophosphorylation activity after the cells were washed free of the compound was dependent on inhibitory potency. Thus, full activity returned immediately after removal of PD 69896 but required 8 hr after exposure to PD 158780. PD 158780 was highly specific for the EGF receptor in Swiss 3T3 fibroblasts, inhibiting EGF-dependent receptor autophosphorylation and thymidine incorporation at low nanomolar concentrations while requiring micromolar levels for platelet-derived growth factor- and basic fibroblast growth factor-dependent processes. PD 158780 inhibited heregulin-stimulated phosphorylation in the SK-BR-3 and MDA-MB-453 breast carcinomas with IC50 values of 49 and 52 nM, respectively, suggesting that the compound was active against other members of the EGF receptor family. The antiproliferative effects of this series of compounds against A431 cells correlated precisely with the inhibitory potency against EGF receptor autophosphorylation. PD 158780 reduced clone formation in soft agar of fibroblasts transformed by EGF, EGF receptor, or the neu oncogene but not ras or raf, further demonstrating its high degree of specificity. Finally, this compound was active against clone formation in several breast tumors having different expression patterns of the erbB family, indicating an anticancer utility in tumors expressing these receptors.

Tyrosine kinase inhibitors. 12. Synthesis and structure-activity relationships for 6-substituted 4-(phenylamino)pyrimido[5,4-d]pyrimidines designed as inhibitors of the epidermal growth factor receptor

A series of 6-substituted 4-anilinopyrimido[5,4-d]pyrimidines has been prepared and shown to be potent inhibitors of the tyrosine kinase activity of the epidermal growth factor receptor (EGFR). These compounds are structurally related to the pyrido[3,2-d]- and pyrido[3,4-d]-pyrimidines previously shown to be EGFR inhibitors. Their structure-activity relationships (SAR) for inhibition of the isolated enzyme more closely resemble those of the [3,2-d] than the [3,4-d] pyridopyrimidine isomers. This suggests the requirement of an aza atom in the 7- but not the 5-position (i.e., a carbon atom in the 5-position) for the enhanced potency shown by 6-N-methylated derivatives in each series. X-ray crystal structures were determined for the three NHMe derivatives 2, 3, and 5c in the pyrido[9,2-d]-, pyrido[3,4-d]-, and pyrimido[5,4-d]-pyrimidine series, respectively. These show that a carbon rather than a nitrogen atom at the 5-position leads to significant conformational changes in the molecule (a longer C5a-C4 bond and a 30 degrees out-of-plane rotation of the phenyl group), due to the requirement to relieve nonbonding interactions between the C5 and N9 protons. Pyrimido[5,4-d]pyrimidine analogues bearing bulky, weakly basic solubilizing side chains linked to the 6-position through a secondary amine generally retained potency both against the isolated enzyme and for inhibition of autophosphorylation of EGFR in intact A431 cells. This agrees with a recent binding model that suggests this general class of compounds binds to EGFR with the 6-position located in an area of comparative bulk tolerance at the entrance to the ATP-binding pocket. While these solubilized pyrimido[5,4-d]pyrimidine analogues were less potent than the NHMe derivative 5c in the isolated enzyme assay, some were considerably superior to 5c (and among the most potent ever reported) as inhibitors of EGFR autophosphorylation in cellular assays.

Structure-activity relationships for 4-anilinoquinazolines as potent inhibitors at the ATP binding site of the epidermal growth factor receptor in vitro

1. Structure-activity relationships are described for the inhibition of the tyrosine kinase activity (phosphorylation of a fragment of phospholipase Cg1) of the epidermal growth factor receptor (EGFR) by 4-anilinoquinazolines. These compounds are competitive inhibitors at the ATP binding site. 2. The preferred side chain is anilino-, substituted at the 3-position with small lipophilic groups. The quinazoline moiety is absolutely required for activity, but substituents on the quinazoline greatly modulate potency, with electron-donating groups favoured. The most potent analogue, the 6,7-dimethoxy derivative (compound 20), has an IC50 of 29 pmol/L and a very high selectivity for the EGFR over other tyrosine kinase enzymes. 3. The present study shows that it is possible to identify small molecules that are very potent, yet highly selective, inhibitors of a single component of the growth signal transduction pathway in cells.

Tyrosine kinase inhibitors. 10. Isomeric 4-[(3-bromophenyl)amino]pyrido[d]-pyrimidines are potent ATP binding site inhibitors of the tyrosine kinase function of the epidermal growth factor receptor

Following the discovery of the very high inhibitory ability of the 4-[(3-bromophenyl)amino]-quinazolines against the tyrosine kinase activity of the epidermal growth factor receptor (EGFR) (e.g., 3, IC50 0.029 nM), four series of related pyrido[d]pyrimidines bearing electron-donating groups at the 6- or 7-positions have been synthesized and evaluated. The compounds were prepared by nucleophilic substitution of the corresponding 6- and 7-fluoro analogues. While members of all series showed potent inhibitory activity against isolated EGFR, there were important differences between the different isomeric pyrido[d]pyrimidines and the parent quinazolines. Overall, the [3,4-d] and [4,3-d] series were the most potent, followed by the [3,2-d] compounds, with the [2,3-d] analogues being least active. Whereas in the parent quinazoline series the addition of steric bulk to a 6- or 7-NH2 substituent (i.e., NHMe and NMe2 groups) dramatically decreased potency, no such trend was discernable in the [3,2-d] series. Furthermore, in the 7-substituted pyrido[4,3-d]- and 6-substituted pyrido[3,4-d]pyrimidine series, and to a limited extent in the 7-substituted pyrido[2,3-d] series, such substitution increased potency dramatically, to the extent that the 7-(methylamino)pyrido[4,3-d]pyrimidine (5f) (IC50 0.13 nM) and 6-(methylamino)pyrido[3,4-d]pyrimidine (7f) (IC50 0.008 nM) constitute important new leads. Selected compounds were evaluated for their ability to inhibit EGFR autophosphorylation in A431 cells, and a positive quantitative correlation was found between this activity and inhibitory activity against the isolated enzyme.

Tyrosine kinase inhibitors. 9. Synthesis and evaluation of fused tricyclic quinazoline analogues as ATP site inhibitors of the tyrosine kinase activity of the epidermal growth factor receptor

Following the discovery of 4-[(3-bromophenyl)amino]-6,7-dimethoxyquinazoline (4; PD 153035) as an extremely potent (IC(50) 0.025 nM) inhibitor of the tyrosine kinase activity of the epidermal growth factor receptor (EGFR), several fused tricyclic quinazoline analogues have been prepared and evaluated for their ability to inhibit the enzyme. The most potent compound was the linear imidazo[4,5-g]quinazoline (8), which exhibited an IC(50) of 0.008 nM for inhibition of phosphorylation of a fragment of phospholipase C-gamma-1 as substrate. While N-methyl analogues of 8 showed similar potency, analogous N-[2-(dimethylamino)ethyl] derivatives were less effective. The next most potent compounds were the linear pyrazoloquinazolines (19 and 20) (IC(50)s 0.34 and 0.44 nM) and pyrroloquinazoline (21) (IC(50) 0.44nM), while several other linear tricyclic ring systems of similar geometry to 8 (triazolo-, thiazolo-, and pyrazinoquinazolines) were less effective. In the imidazo[4,5-g]quinazoline and pyrroloquinazoline series, the corresponding angular isomers were also much less effective than the linear ones. These results are consistent with structure-activity relationship studies previously developed for the 4-[(3-bromophenyl)amino] quinazolines, which suggested that small electron-donating substituents at the 6- and 7-positions were desirable for high potency. Cellular studies of the linear imidazoloquinazoline 8 show that it can enter cells and rapidly and very selectively shut down EGF-stimulated signal transmission by binding competitively at the ATP site of the EGFR.

Tyrosine kinase inhibitors. 8. An unusually steep structure-activity relationship for analogues of 4-(3-bromoanilino)-6,7-dimethoxyquinazoline (PD 153035), a potent inhibitor of the epidermal growth factor receptor

4-(3-Bromoanilino)-6,7-dimethoxyquinazoline (32, PD 153035) is a very potent inhibitor (IC50 0.025 nM) of the tyrosine kinase activity of the epidermal growth factor receptor (EGFR), binding competitively at the ATP site. Structure-activity relationships for close analogues of 32 are very steep. Some derivatives have IC50s up to 80-fold better than predicted from simple additive binding energy arguments, yet analogues possessing combinations of similar phenyl and quinazoline substituents do not show this "supra-additive" effect. Because some substituents which are mildly deactivating by themselves can be strongly activating when used in the correct combinations, it is proposed that certain substituted analogues possess the ability to induce a change in the conformation of the receptor when they bind. There is some bulk tolerance for substitution in the 6- and 7-positions of the quinazoline, so that 32 is not the optimal inhibitor for the induced conformation. The diethoxy derivative 56 [4-(3-bromoanilino)-6,7-diethoxyquinazoline] shows an IC50 of 0.006 nM, making it the most potent inhibitor of the tyrosine kinase activity of the EGFR yet reported.

Tyrosine kinase inhibitors. 5. Synthesis and structure-activity relationships for 4-[(phenylmethyl)amino]- and 4-(phenylamino)quinazolines as potent adenosine 5'-triphosphate binding site inhibitors of the tyrosine kinase domain of the epidermal growth factor receptor

A series of 4-substituted quinazolines and related compounds have been prepared and evaluated for their ability to inhibit the tyrosine kinase activity of the epidermal growth factor receptor on a phospholipase C-gamma 1-derived substrate. The results show a narrow structure-activity relationship (SAR) for the basic ring system, with quinazoline being the preferred chromophore and benzylamino and anilino the preferred side chains. In the 4-anilino series, substitution on the 3-position of the phenyl ring with small lipophilic electron-withdrawing groups provided analogues with enhanced potency. Two series of compounds [4-(phenylmethyl)amino and 4-(3-bromophenyl)amino] were studied to determine SARs for quinazoline substituents. In the more active 4-(3-bromophenyl)amino series, electron-donating groups (NH2, OMe) at the 6- or 7-position increased activity, in a pattern consistent with a requirement for high electron density in the vicinity of the 8-position of the quinazoline ring. The 6,7-dimethoxy derivatives were the most effective in both series, with the 4-(3-bromophenyl)amino derivative (3) having an IC50 of 0.029 nM, making it by far the most potent reported inhibitor of the tyrosine kinase activity of the epidermal growth factor receptor enzyme.

Metabolism and elimination of 5,6-dimethylxanthenone-4-acetic acid in the isolated perfused rat liver

5,6-Dimethylxanthenone-4-acetic acid (DXAA) is a synthetic xanthenone derivative that is active against murine solid tumors and is being formulated for clinical trials. This study used the isolated perfused rat liver to compare the hepatic metabolism and biliary excretion of DXAA with flavone-8-acetic acid (FAA), a synthetic flavonoid undergoing clinical evaluation as an anticancer drug. Perfusate, bile, and liver samples were assayed for parent drug and metabolites by HPLC. Three FAA metabolites were present in bile, and one of these coeluted with FAA acyl glucuronide. Alkaline hydrolysis of high-dose DXAA bile samples resulted in the disappearance of 5 of 7 metabolite peaks. One biliary metabolite was identified by mass spectrometry as the acyl glucuronide and its presence in bile accounted for > 50% of the DXAA dose. A second compound that was resistant to alkaline hydrolysis was characterized as a hydroxylated DXAA metabolite. A total of 28% of the high dose DXAA was recovered unchanged in the perfusate, liver, and bile, compared with 11% of the low dose DXAA and 40% of the FAA dose. Protein binding of DXAA in perfusate was saturable, ranging from 94.5% at 112 microM to 72.4% at 1125 microM, whereas binding in human plasma was > 99% at concentrations between 11.5 and 1243 microM. This study demonstrates that DXAA undergoes extensive acyl glucuronidation followed by biliary excretion in the isolated perfused rat liver. Its hepatic metabolism may be saturable, and DXAA seems to be more extensively metabolized than FAA. Finally, DXAA protein binding in human plasma is high and not dose-dependent at concentrations likely to be clinically relevant.

Tyrosine kinase inhibitors. 4. Structure-activity relationships among N- and 3-substituted 2,2'-dithiobis(1H-indoles) for in vitro inhibition of receptor and nonreceptor protein tyrosine kinases

A series of 3-substituted 2,2'-dithiobis(1H-indoles) were synthesized and evaluated for their ability to inhibit the tyrosine kinase activity of both the epidermal growth factor receptor (EGFR) and the nonreceptor pp60v-src tyrosine kinase, to extend the available structure-activity relationships for this series. The majority of the compounds were prepared either by reaction of 2-chloro-1-methylindole-3-carbonyl chloride with amines, followed by thiomethylation, demethylation, and oxidative dimerization, or by reaction of isocyanates with the anion of 1-methyl-2-indolinethione followed by dimerization. Overall, inhibitory activity is retained by analogues having a wide variety of side chains. A series of 3-carboxamide analogues had moderate to good activity against isolated EGFR (IC50s 1-20 microM), with monoalkyl substitution of the carboxamide being optimal. Polar side chains were generally less effective than lipophilic ones, with benzyl being particularly effective. However, N,N-disubstitution was the most effective pattern for inhibition of pp60v-src. A variety of substituted N-phenylcarboxamides had lower activity against EGFR than the parent derivative, and a N-thienylcarboxamide also had low activity. A series of 3-ketones, including methyl, phenyl, and furyl derivatives, showed moderate activity against the pp60v-src kinase, but were less effective against EGFR. The mechanism of inhibition of both kinases by these drugs was shown to be noncompetitive with respect to both ATP and peptide substrate. Selected compounds inhibited the growth of Swiss 3T3 cells with IC50s in the low micromolar range and inhibited bFGF-mediated intracellular tyrosine phosphorylation in the same cell line. Thiol inhibits the effects of the compounds, suggesting that one possible mechanism of inhibition is thiol-disulfide exchange with thiol-containing residues in the catalytic sites. Crystal structures of two representative compounds show a folded, V-shaped structure, with the disulfide bridge exposed, consistent with this hypothesis.

Disposition of the novel antitumour agent xanthenone-4-acetic acid in the mouse: identification of metabolites and routes of elimination

1. Xanthenone-4-acetic acid (XAA) is an experimental antitumour agent which resembles flavone-8-acetic acid in its induction of cytokine synthesis, nitric oxide production and tumour haemorrhagic necrosis. We have investigated the excretion and metabolic fate of XAA in the BDF1 mouse. 2. XAA was administered intravenously at the maximal tolerated dose (1090 mumol/kg). Urine, plasma and bile were collected and subjected to analysis by hplc. Urine samples demonstrated labile metabolites which released XAA following incubation with beta-glucuronidase/sulphatase or at pH 9.0. The structures of isolated XAA metabolites were characterized by ms or 1H-NMR spectra at 400 MHz. 3. The major metabolite pathway of XAA involves conjugation with glucuronic acid, since the resulting metabolite, XAA acyl glucuronide, accounts for 25% of the dose excreted in the urine. Other metabolite pathways include alpha-oxidation of the acetic acid side chain and aromatic hydroxylation of the xanthenone ring.

Tyrosine kinase inhibitors. 3. Structure-activity relationships for inhibition of protein tyrosine kinases by nuclear-substituted derivatives of 2,2'-dithiobis(1-methyl-N-phenyl-1H-indole-3-carboxamide)

A series of indole-substituted 2,2'-dithiobis(1-methyl-N-phenyl-1H-indole-3-carboxamides) were prepared and evaluated for their ability to inhibit the tyrosine kinase activity of both the epidermal growth factor receptor (EGFR) and the nonreceptor pp60v-src tyrosine kinase. The compounds were synthesized by conversion of appropriate 1-methyloxindoles to 1-methyl-2-indolinethiones with P2S5 followed by subsequent reaction with NaH and phenyl isocyanate and oxidative dimerization of the resulting 2,3-dihydro-N-phenyl-2-thioxo-1H-indole-3-carboxamides. The parent compound and many of the substituted analogues were moderately potent inhibitors of both kinase enzymes, but no clear relationships were seen between substitution on the indole ring and inhibitory activity. While 4-substituted compounds were generally inactive, 5-substituted derivatives with electron-withdrawing groups showed inhibitory activity. However, none of the substituted compounds showed significantly better activity than the unsubstituted parent compound. There was generally a good correlation between activity against the EGFR and pp60v-src kinases, but several compounds did show some specificity (> 20-fold) of inhibition; 5-Cl and 5-Br derivatives preferentially inhibited pp60v-src, while the 5-CF3 compound preferentially inhibited EGFR. Selected compounds from the series were found to inhibit the growth of Swiss 3T3 fibroblasts with IC50S in the range 2-25 microM, the most active being 4-substituted derivatives. The compounds inhibited bFGF-mediated protein tyrosine phosphorylation in intact cells more effectively than EGFR- or PDGF-mediated phosphorylation.

Tyrosine kinase inhibitors. 1. Structure-activity relationships for inhibition of epidermal growth factor receptor tyrosine kinase activity by 2,3-dihydro-2-thioxo-1H-indole-3-alkanoic acids and 2,2'-dithiobis(1H-indole-3-alkanoic acids)

A series of 2,3-dihydro-2-thioxo-1H-indole-3-alkanoic acids, and their methyl esters were prepared, the majority by oxidation of 1H-indole-3-alkanoic acids (DMSO/HCl), followed by thiation of the corresponding 2,3-dihydro-2-oxo-1H-indole-3-alkanoic acid esters. The monomeric thiones undergo facile and reversible oxidation to the corresponding 2,2'-dithiobis(1H-indole-3-alkanoic acids). The compounds were evaluated for their abilities to inhibit the tyrosine kinase activity of the epidermal growth factor receptor using a native complex contained in plasma membrane vesicles shed from cultured A431 cells, and to inhibit the growth of Swiss 3T3 mouse fibroblast in culture. Enzyme inhibitory activity is dependent on the length of the side chain, with propanoic acid derivatives showing the highest activity. The acids are generally significantly more potent than the corresponding esters, and the disulfides more active than the corresponding monomers. An ability to undergo the thione-thiol tautomerism necessary for dimerization is essential, with 3,3-disubstituted compounds being inactive. Overall, the data suggest that the disulfide is the more active form, with much of the activity of the monomeric thiones being due to varying degrees of conversion to the disulfide during the assay. In the growth inhibition assay, the methyl esters are more potent than their corresponding carboxylic acids, and the dimers are generally more potent than the monomers. The data show these compounds to be a novel and potent class of inhibitors of epidermal growth factor receptor tyrosine kinase activity.

Structure-activity relationships for substituted 9-oxo-9,10-dihydroacridine-4-acetic acids: analogues of the colon tumour active agent xanthenone-4-acetic acid

A series of 9-oxo-9,10-dihydroacridine-4-acetic acids (acridone-4-acetic acids) were prepared by Jourdan-Ullmann condensation of 2-halobenzoic acids with 2-aminophenylacetic acids, followed by H2SO4-induced cyclodehydration of the resulting 2-[2-(carboxymethyl)phenylamino]benzoic acids. These were evaluated for their ability to induce haemorrhagic necrosis in transplanted colon 38 tumours in mice, using a short-term histology assay. The results broadly paralleled those seen previously for xanthenone-4-acetic acids, with 1-, 2- and 7-substitution being dystherapeutic, and substitution at the 5- and 6-positions by lipophilic groups increasing activity. While some analogues were as active as xanthenone-4-acetic acids in the histology assay and gave significant growth delays against colon 38 tumours in vivo, as a class the 9-oxo-9,10-dihydroacridine-4-acetic acids were generally less potent than the xanthenone-4-acetic acids.