Medicine Branch, National Cancer Institute, NIH, Bethesda, MD and Department of Medicine, New York Medical College, Valhalla, NY, USA

Correspondence to: M V Blagosklonny, Medicine Branch, Bldg 10 Room 12 N 226, NIH, Bethesda, MD 20892, USA; Fax: (301) 402 0172

Seemingly disappointing, the Bcr-Abl kinase inhibitor STI-571 shares an 'unfortunate' characteristic with conventional cancer drugs: the development of drug resistance. I argue that the resistance must develop even faster to STI-571 than to conventional drugs, because STI-571 is so effective. This is predictable, but is it inevitable? And how do mechanisms of resistance in relapse depend on a degree of remission. In addition to mutation rate and number of tumor cells, one additional factor determines relapse vs 'extinction' of the leukemia cell population.

Leukemia (2002) 16, 570-572. DOI: 10.1038/sj/leu/2402409

STI-571; Bcr-Abl; leukemia; drug resistance; selective therapy

'Unfortunate' characteristic of STI-571

STI-571 is an inhibitor of the Bcr-Abl oncogene kinase.¹ This kinase is etiologic for adult chronic myeloid leukemia (CML) and blast crisis as well as a fraction of acute lymphoblastic leukemias.² As a product of the fusion gene which is absent in normal cells, Bcr-Abl represents an ideal therapeutic target. In fact, therapy with STI-571 which inhibits the Abl kinase induces remissions. Furthermore, adverse effects of STI-571 are minimal. Complete hematologic responses were observed in 53 of 54 patients with CML and typically occurred in the first 4 weeks of therapy.³ Also, responses occurred in 55% of patients with a myeloid blast crisis.⁴

Disappointingly, however, the remissions in blast crisis patients usually last only 2-6 months. Despite continuous therapy, relapses occur due to an acquired drug resistance. A recent report in Science by Gorre et al⁵ indicates that amplifications and mutations in Bcr-Abl renders cells resistant to STI-571. As stated by Jean Marx 'STI-571 shares an unfortunate characteristic with conventional cancer drugs': the development of drug resistance.⁶ As further explained by McCormick,⁷ unlike other existing chemotherapies such as 5-fluorouracil and methotrexate which block enzymes that are indirectly involved in cancer, STI-571 affects a target that directly causes leukemia. Yet, despite this fundamental difference, resistance to STI-571 is depressingly similar to resistance to 5-FU and methotrexate.⁷

Every cloud has a silver lining. Although it may sound paradoxical, the rapid development of resistance to STI-571 indicates that this drug is effective and selective.

The Darwinian selection and STI-571

If STI-571 is truly selective and effective, such an unfortunate characteristic as the development of drug resistance is predictable. In this respect, STI-571 must be 'worse' than conventional drugs: the resistance to STI-571 must develop faster and be more uniform than the resistance to conventional (and less selective) drugs. In light of the Darwinian selection, by killing drug-sensitive cells, a drug selects for resistant clones (Figure 1). This is exactly what a selective therapy does by definition (Figure 1). Simultaneously, a drug also selects for genetically unstable cells (cells with high mutation rate due to defects of DNA repair).⁸ Like any mutation, mutations that confirm resistance (eg mutation in Bcr-Abl) predominantly occur in genetically unstable cells. As shown in Figure 1, selection for such mutations automatically selects for genetically unstable cells. In other words, by killing drug-sensitive cells, a drug selects for resistant cells which are genetically unstable.

Three requirements for development of resistance to highly effective drugs

In order to be selected, resistance-confirming mutations must first occur. Will a mutation repertoire always provide resistant variants? There are three factors that determine development of the necessary resistance to highly selective drugs: (1) genetic instability of tumor cells; (2) number of self-repopulating tumor cells; and (3) the compatibility of mechanisms of resistance with cell functions.

Genetic instability (a mutation rate)

Like any mutations, resistance-confirming mutations occur more often in genetically unstable cells. Given that genetic instability is a hallmark of cancer,⁹ a high mutation rate is (to a certain extent) an inevitable predisposition. Currently, we cannot therapeutically enhance DNA repair to prevent resistance-bearing mutations.

As shown in Figure 1, anticancer drugs select for cells with a high mutation rate.⁸ This suggests that prior chemotherapy, which selects for genetic instability, increases the chances of relapse following STI-571 therapy. This may explain why pre-treated patients relapse faster. On the other hand, one can expect that STI-571-relapsed leukemia will be even more genetically unstable. These predictions are testable.

Number of tumor cells (tumor burden)

Obviously, more cells can produce more mutations and a wider mutation repertoire. Therefore, a high number of proliferating tumor cells, or high tumor burden, will accelerate the development of drug resistance. Patients with a high tumor burden will be more likely to relapse. This predicts that patients in the early stages of leukemia or in remission caused by chemotherapy may benefit from STI-571 most. In contrast to genetic instability, tumor burden can be decreased by chemotherapy, radiotherapy or surgically.

Why do patients in blast crisis relapse?

Certainly, progression to blast crisis can be accompanied by acquiring resistance due to loss of tumor suppressors and activation of oncogenes, including Bcr-Abl amplifications. In fact, only 55% of patients with a myeloid blast crisis responded.⁴ However, if blast crisis undergoes remissions, this indicates sensitivity to therapy. Furthermore, it has been shown that cells in blast crisis and in chronic phase are equally sensitive to STI-571 in vitro.¹⁰ So why do patients in blast crisis have relapsed leukemia after an initial response?

A high number of proliferating blast cells and increased genetic instability can explain why resistance develops more easily in blast crisis than in chronic phase. (1) There is a relatively low number of blast repopulating cells in chronic phase. (2) Genetic instability is also higher in blast crisis. In fact, in the blast phase, more than 60% of patients show additional cytogenetic changes, consistent with genome instability.¹¹ Genetic instability universally rises during tumor progression.⁹ Like anticancer drugs, barriers in tumor progression select for additional mutations, and these mutations happen to occur in genetically unstable cells (Figure 2). Therefore, the mutation rate rises during tumor progression⁸ (from CML to blast crisis).

Accordingly, while sensitivity to a drug determines response and remission, it may not predict relapse. Genetic instability and initial tumor burden may determine relapse.

Mechanism of resistance

Different and numerous potential mechanisms of resistance increase the probability of its development. Expression of drug pumps, mutations or amplifications in drug targets, and inhibition of downstream apoptotic pathways can confer drug resistance. In vitro, amplification of Bcr-Abl,^12,13 an increase in the Bcr-Abl protein expression,¹⁴ expression of Pgp,¹³ and activation of downstream signaling pathways¹⁵ can confer drug resistance. Amplifications of Bcr-Abl, duplicated and inverted Ph chromosome were found in relapse.^5,16,17,18 Given that leukemogenic effects of Bcr-Abl are proportional to its levels,¹⁹ development of STI-571 resistance due to Bcr-Abl overexpression may further drive leukemogenesis. As reported by Gorre et al,⁵ amplification of Bcr-Abl and the mutation, threonine to isoleucine substitution at position 315 of c-Abl (T315I), lead to resistance in most patients. In other clinical trials by Hochhaus, Barthe et al, however, this particular mutation was not found, although Glu to Lys substitution at position 255 of Abl (E255K) was found in two patients.¹⁸ What might determine the difference? As noted by Gorre et al, the patients, who later acquired T315 mutation, obtained complete remissions. This indicates that most Bcr-Abl cells were killed, and perhaps one single cell with the utmost resistance-confirming mutation (eg T315I) has produced a resistant clone. The relapsed resistant population is relatively uniform (Figure 3a), as a benchmark of highly selective and effective therapy. The patients with partial response will have a high proportion of sensitive cells, as well as a diverse population of resistant cells (Figure 3b).

Finally, the mechanism of resistance must be compatible with cell functions. It seems that two mutations are emerging: T315I and E255K. By combining STI-571 with drugs that target Bcr-Abl by a different mechanism, we may decrease the chance for leukemia cells to acquire resistance. Several drugs that hit different parts of the same target might be ideal.⁷ For example, geldanamycin, an Hsp90-active drug, targets Bcr-Abl for degradation and depleted cells from Bcr-Abl thus causing apoptosis and rendering cells sensitive to chemotherapy.^20,21

The kinase may not tolerate additional alterations.⁷ Mutations may never provide such a drug-resistant Bcr-Abl variant. Despite a need for resistance, cells may not produce the necessary mechanism. Then, therapy will force leukemia cells to die. As put by Richard Dawkins, in his discussion of the evolution of species by a means of natural selection, 'it is not true that whatever selection might in principle favor, mutation will always come up with the necessary variation. No animal breathes fire out of its nostrils like a dragon, for instance'.²²

1 Druker BJ, Lydon NB. Lessons learned from the development of an Abl tyrosine inhibitor for chronic myelogenous leukemia. J Clin Invest 2000; 105: 3-7. MEDLINE

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4 Druker BJ, Sawyers CL, Kantarjian H, Resta DJ, Reese SF, Ford JM, Capdeville R, Talpaz M. Activity of a specific inhibitor of the BCR-ABL tyrosine kinase in the blast crisis of chronic myeloid leukemia and acute lymphoblastic leukemia with the Philadelphia chromosome. N Engl J Med 2001; 344: 1038-1042. MEDLINE

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6 Marx J. Cancer research. Why some leukemia cells resist STI-571. Science 2001; 292: 2231-2233. MEDLINE

7 McCormick F. New-age drug meets resistance. Nature 2001; 412: 281-282. Article MEDLINE

8 Blagosklonny MV. How carcinogens (or telomere dysfunction) induce genetic instability: associated-selection model. FEBS Lett 2001; 506: 169-172. MEDLINE

9 Lengauer C, Kinzler KW, Vogelstein B. Genetic instability in human cancer. Nature 1998; 396: 643-649. Article MEDLINE

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11 Ohyashiki K, Iwama H, Tauchi T, Shimamoto T, Hayashi S, Ando K, Kawakubo K, Ohyashiki JH. Telomere dynamics and genetic instability in disease progression of chronic myeloid leukemia. Leuk Lymphoma 2000; 40: 49-56. MEDLINE

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17 Mohammed M, Shin S, Deng S, Ford J, Paquette RL, Sawyers CL. BCR/ABL gene amplification: a possible mechanism of drug resistance in patients treated with an ABL specific kinase inhibitor. Blood 2000; 96: Abstr. 1486.

18 Hochhaus A, Kreil S, Corbin A, La Rosee P, Lahaye T, Berger U, Cross NC, Linkesch W, Druker BJ, Hehlmann R, Gambacorti-Passerini C, Corneo G, D'Incalci M, Barthe C, Cony-Makhoul P, Melo JV, Mahon JR, Gorre M, Shah N, Ellwood K, Nicoll J, Sawyers CL. Roots of clinical resistance to STI-571 cancer therapy. Science 2001; 293: 2163. MEDLINE

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20 Nimmanapalli R, O'Bryan E, Bhalla K. Geldanamycin and its analogue 17-allylamino-17-demethoxygeldanamycin lowers Bcr-Abl levels and induces apoptosis and differentiation of Bcr-Abl-positive human leukemic blasts. Cancer Res 2001; 61: 1799-1804. MEDLINE

21 Blagosklonny MV, Fojo T, Bhalla KN, Kim J-S, Trepel JB, Figg WD, Rivera Y, Neckers LM. The Hsp90 inhibitor geldanamycin selectively sensitizes Bcr-Abl-expressing leukemia cells to cytotoxic chemotherapy. Leukemia 2001; 15: 1537-1543. MEDLINE

22 Dawkins R. The Blind Watchmaker W Norton & Co: New York, 1986.

Figure 1 Effective therapy selects for both resistance-confirming mutation and genetic instability. Resistance-confirming (adaptive) mutations occur in genetically unstable cells.

Figure 2 Tumor progression selects for genetic instability. Tumor-promoting (adaptive) mutations occur in genetically unstable cells.

Figure 3 Resistance profile of leukemia cell population in relapse. (a) Following complete remission; (b) following partial remission.