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Signaling Inhibitors in the Clinic: New Agents and New Challenges

Fox Chase Cancer Center, Philadelphia, PA

PHASE I TRIALS are an essential component of clinical cancer research, providing initial toxicology, pharmacology, and dosing recommendations necessary for subsequent phase II and III investigations. While numerous reports of phase I trials are submitted to the Journal of Clinical Oncology, only a minority can be accepted for publication. Those accepted usually describe trials of agents that have novel mechanisms of action or preliminary activity that indicates significant therapeutic promise, often in conjunction with sound pharmacokinetic data. One such area of therapeutic potential is signal transduction inhibition.

Delineation of signaling pathways that link growth factors and growth factor receptors at the cell periphery to the genetic programs for proliferation and differentiation contained in the nucleus has spawned a remarkable increase of new agents aimed at molecular targets within these pathways.¹ Mutation leading to continuous activation, or amplification and overexpression of signaling proteins, most of which have been identified as cellular oncogenes, usually confers a growth or survival advantage to cells. Thus, the ability to interfere with cell signaling, whether at the level of growth factor-receptor interaction, receptor activation, or at protein targets distal to the receptor, such as ras and raf, has both therapeutic and chemopreventive potential. While basic understanding of the complex tapestry of cell signaling is evolving, clinical investigation of signal transduction inhibition is well underway.

The phase I study of SU101 reported by Eckhardt et al² in this issue of the Journal is among the first clinical investigations of small molecule inhibitors of signal transduction. SU101 is an isoxazole derivative that inhibits the platelet-derived growth factor receptor (PDGF-R)/Flk-1 family of receptor tyrosine kinases. Receptor tyrosine kinases (RTKs), such as the PDGF-R, epidermal growth factor receptor, and insulin-like growth factor receptors, are activated when the appropriate growth factor (ligand) binds extracellular portions of the receptor, causing receptor oligerization. Oligerization results in conformational changes of receptor cytoplasmic domains, which in turn increase tyrosine kinase activity. Activated RTKs phosphorylate tyrosine moieties within the receptor (autophosphorylation) and on a variety of intracellular substrates recognized by specific amino acid sequences. Conversely, autophosphorylation of RTKs allows their recognition by docking proteins that further link the membrane signal to more distant members of the signaling cascade.³

In preclinical studies, inhibition of PDGF beta subtype RTK (PDGFß-RTK) by SU101 was associated with decreased PDGF-stimulated DNA synthesis and cell cycle progression and with inhibition of cell growth.⁴ Additional studies demonstrated that SU101 given intraperitoneally or intracerebrally delayed tumor growth in mice bearing rat and human tumor xenografts and in a rat orthotopic brain tumor model. These effects were greatest in cells engineered to overexpress PDGFß-R and in tumors that strongly express PDGFß-R. Because several human tumors, including gliomas and breast carcinomas, strongly express PDGF-R, the rationale for clinical testing of this molecule is strong.

A key aspect of SU101 pharmacology is rapid conversion of the drug by intramolecular rearrangement to SU0020, the major metabolite in rat and human plasma. In contrast to the parent drug, SU0020 is an inhibitor of dihydro-orotate dehydrogenase, blocking de novo pyrimidine synthesis. This mechanism of action, distinct from that of the parent molecule, could also contribute to the antiproliferative activity of SU101.³ In preclinical studies, the two mechanisms of action were distinguished by adding excess uridine to circumvent the antimetabolite effect of SU0020.^4,5 However, inhibition of dihydro-orotate dehydrogenase alone as a mechanism of action does not engender much enthusiasm, because other inhibitors of de novo pyrimidine synthesis (eg, acivicin, brequinar, and N-phosphonoacetyl-L-aspartate) have not been active in the clinic.

These and other issues are relevant to how we interpret the results reported by Eckhardt et al. In their carefully conducted study, SU101 was administered as a 24-hour infusion weekly for 4 consecutive weeks out of each 6-week cycle. A maximum-tolerated dose was not reached because the large fluid volume required for administration of the drug prevented escalation to dose-limiting toxicity. Although toxicity is the conventional surrogate of biologic activity in phase I trials, pharmacokinetic or pharmacodynamic end points based upon preclinical studies that relate a range of drug concentrations to antitumor activity are reasonable alternatives. This strategy was used by Eckhardt et al in the selection of a dose of SU101 for subsequent clinical trials.

Ideally, concentrations of SU101 required for antitumor effects in vitro or in an animal model would be compared with those achieved in the clinic in order to determine the optimal dose for efficacy trials. For several reasons, including rapid conversion to SU0020, SU101 was detected only at low concentrations and transiently in some patients, and not at all in others. Instead, Eckhardt et al have used plasma concentrations of the metabolite SU0020 to guide selection of a biologically active dose of SU101 for further clinical studies. At doses of SU101 ranging from 225 to 443 mg/m², the observed trough concentrations of SU0020 were greater than those known to inhibit growth of cells but most likely by PDGF-independent mechanisms. The obvious problem with this approach is that SU0020 is not SU101—the parent drug and the metabolite differ in mechanism of action and pharmacologic properties. Although SU0020 may have antiproliferative or cytotoxic effects by PDGF-independent mechanisms, it has not been shown to inhibit PDGFß-RTK. Consequently, it is not possible to conclude that selection of SU101 dose on the basis of plasma concentrations of SU0020 will lead to a fair test of PDGFß-RTK inhibition in efficacy trials.

An even stronger basis for determining that relevant doses of SU101 were achieved would be demonstration of PDGFß-RTK inhibition in normal or tumor cells. This finding would establish that the desired effect and putative mechanism of action occurred at the tissue level. However, these pharmacodynamic studies require that adequate samples of tumor or a nonmalignant, but relevant, surrogate tissue be available, and that PDGFß-RTK activity can be reliably quantified in the samples. These conditions are seldom realized in the world of solid tumors but would be worth pursuing, even if accomplished in only a limited number of patients for proof of principle.

Concern over rapid conversion of SU101 to a metabolite that inhibits de novo pyrimidine synthesis should temper enthusiasm for this drug as an inhibitor of PDGFß-RTK. On the other hand, Eckhardt et al observed an objective response in a patient with malignant glioma, a tumor type characterized by high levels of PDGFß-RTK. If phase II trials demonstrate significant activity of SU101 in glioma or other tumors, then concern over mechanism of action will be reduced but not eliminated. As the authors acknowledge, it will be crucial for the future development of this agent to determine whether and to what extent SU101 inhibits PDGFß-RTK activity in human tissues. Regardless of what subsequent trials of SU101 show, there will continue to be strong rationale for clinical trials of inhibitors of PDGFß-RTK and other RTKs.

Finally, Eckhardt et al discuss several issues concerning clinical trials of drugs that may have cytostatic rather than cytotoxic mechanisms of action. How do we identify activity if not by tumor regression? Will phase III trials using time to progression or survival be required, even if more costly, to detect activity? If a molecularly targeted therapy demonstrates greater activity in cells that overexpress the target of interest, should accrual to clinical trials be limited to patients with tumors most likely to express or overexpress the target? More fundamentally, the pharmacologic behavior of SU101 observed by Eckhardt et al leaves open to question how this drug works in humans. Should we consider SU101 a novel inhibitor of PDGF-dependent signaling, an antimetabolite, or a combination of the two? Are the issues of clinical trial design raised by Eckhardt et al relevant to SU101? It is hoped that additional clinical studies with pharmacodynamic end points will provide clarification. In the meanwhile, we should remain open-minded regarding the mechanism of action of SU101 and other novel therapeutics.

REFERENCES

1. Barinaga M: From bench top to bedside. Science 278:1036-1039, 1997[Free Full Text]

2. Eckhardt SG, Rizzo J, Sweeney KR, et al: Phase I and pharmacologic study of the tyrosine kinase inhibitor SU101 in patients with advanced solid tumors. J Clin Oncol 17:1095-1104, 1999[Abstract/Free Full Text]

3. Pawson T: Protein modules and signaling networks. Nature 373:573-580, 1995[Medline]

4. Shawver LK, Schwartz DP, Mann E, et al: Inhibition of platelet-derived growth factor-mediated signal transduction and tumor growth by N-[4-(Trifluroromethyl)-phenyl]5-methylisoxazole-4-carbamide. Clin Cancer Res 3:1167-1177, 1997[Abstract]

5. Shawver LK, Sutton B, West KA, et al: The anti-tumor efficacy of SU101 in human tumor models is exerted by the parent compound and not the metabolite. Proc Am Assoc Cancer Res 39:320, 1998 (abstr 2185)

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