The genomic era has shifted anticancer drug development from its traditional mode concentrated on natural product cytotoxic agents to mechanism-based drug design focused on signal transduction pathways. Yet traditional cytotoxic chemotherapies continue to have an important role in the armamentarium. This is particularly true when one considers that important elements of solid tumor physiology – acidosis and hypoxia – have rarely been incorporated into algorithms for anticancer drug development. It is now well established that a majority of solid tumors exist in an acidic and hypoxic microenvironment that promotes resistance to radiation and chemotherapies apart from any drug-induced target mutations or efflux protein pumps. The acidic extracellular environment leads to a pH gradient unique to tumor cells. This gradient will favor uptake and retention of small molecule drugs that are weak acids. The converse is true for weak bases. The camptothecin class of topoisomerase I inhibitors is one example of a natural product cytotoxic that can exploit the tumor pH gradient. Screening of compounds based on selective activity at acidic pH (pH modulation), rather than potency, reveals analogs that are over ten times more active under the acidic conditions prevalent in vivo. Thus, knowledge of the tumor metabolic phenotype gained at the beginning of the 20(th) century can lead to more effective anticancer drugs in the new millennium.

Non-small cell lung cancer (NSCLC) is an aggressive disease that is generally resistant to chemotherapy. As a result, the prognosis for patients with NSCLC is poor. Currently, platinum-based regimens are the standard of care for patients with advanced NSCLC. However, these regimens are associated with severe and often cumulative hematologic and nonhematologic toxicities, limiting dose intensity. Therefore, novel chemotherapeutic agents and combination regimens may improve the outcome for these patients. A variety of new agents and combinations have been investigated in the treatment of NSCLC. However, to date, no clearly superior single-agent or combination regimen has emerged. Topotecan (Hycamtin; GlaxoSmithKline; Philadelphia, PA), a topoisomerase I inhibitor, is currently approved for the treatment of patients with relapsed small cell lung cancer (SCLC) and is associated with manageable, noncumulative, hematologic toxicities. In addition, topotecan demonstrates a favorable nonhematologic tolerability profile compared with agents currently used in the treatment of patients with NSCLC. The success of topotecan in patients with SCLC has made it an attractive option in the NSCLC setting. Topotecan-based combination regimens in the first-line treatment of NSCLC have demonstrated promising antitumor activities with favorable toxicity profiles. Many topotecan combination regimens have induced stable disease, a response that may offer meaningful clinical benefit in the palliative treatment of patients with advanced disease. Topotecan plus gemcitabine (Gemzar; Eli Lilly and Company; Indianapolis, IN) and single-agent topotecan may be particularly appropriate for patients in the second-line setting, in which palliation of symptoms is an important outcome of chemotherapy. Herein, the future role of topotecan in the first- and second-line treatment of NSCLC and the potential role of resistance mechanisms obtained from in vivo dose-response studies in designing future combination regimens are discussed.

Natural and synthetic indolocarbazole compounds have triggered considerable interest since the discovery in 1986 of the inhibitory properties of staurosporine toward protein kinase C (PKC). Later, it has been shown that indolocarbazole compounds may inhibit various kinases, such as cyclin dependent-kinases and/or topoisomerase I, someones behave only as DNA intercalators. In this review are presented various indolocarbazole compounds bearing a sugar moiety and their biological targets. The relevance of these targets to develop indolocarbazole compounds as potential antitumor agents is discussed.

While the majority of topoisomerase (topo) inhibitors show selectivity against either topo I or topo II, a small class of compounds can act against both enzymes. These can be divided into three classes. The first and largest class comprise drugs that bind to DNA by intercalation and include the clinically-evaluated acridine DACA, the benzopyridoindole intoplicine, the indenoquinolinone TAS-103, the benzophenazine XR11576, and the pyrazoloacridine NSC 366140. The second category comprises hybrid molecules, prepared by physically linking separate inhibitors of topo I and topo II, or by linking pure topo inhibitors to other DNA-interactive carriers. While several derivatives (e.g., camptothecin-epipodophyllotoxin and ellipticine-distamycin hybrids) have been prepared, there have been no detailed studies. The third category are less well defined as a structural class, but apparently recognize structural motifs that are present in both topo I and II enzymes. These include a series of benzoisoquinolinium quaternary salts such as NK 109, and more interestingly modified versions of classical topo I or topo II inhibitors; e.g., the modified camptothecin BN 80927 and the modified epipodophyllotoxin tafluposide (F-11782). There is as yet no detailed understanding of the factors that result in selective or dual inhibition, but structure-activity studies in several classes show that structural changes can influence topo I/II selectivity. DNA intercalation mode also appears to play a part. The basis for the high antitumor activity of some topo inhibitors is not yet understood but may depend on the complex pattern of activities that include both inhibition and poisoning of the two enzymes.

Camptothecin (CPT) class of compounds has been demonstrated to be effective against a broad spectrum of tumors. Their molecular target has been firmly established to be human DNA topoisomerase I (topo I). CPT inhibits topo I by blocking the rejoining step of the cleavage/religation reaction of topo-I, resulting in accumulation of a covalent reaction intermediate, the cleavable complex. The primary mechanism of cell killing by CPT is S-phase-specific killing through potentially lethal collisions between advancing replication forks and topo-I cleavable complexes. Collisions with the transcription machinery have also been shown to trigger the formation of long-lived covalent topo-I DNA complexes, which contribute to CPT cytotoxicity. Two novel repair responses to topo-I-mediated DNA damage involving covalent modifications of topo-I have been discovered. The first involves activation of the ubiquitin/26S proteasome pathway, leading to degradation of topo-I (CPT-induced topo-I downregulation). The second involves SUMO conjugation to topo-I. The potentials roles of these new mechanisms for repair of topo-I-mediated DNA damage in determining CPT sensitivity/resistance in tumor cells are discussed.

The standard treatment of multiple myeloma is systemic chemotherapy. Despite 30 years of drug development in myeloma, there are no new drug regimens significantly superior to melphalan and prednisone. In addition, phase II studies of new drugs in myeloma have been disappointing, with low response rates and no prolongation in survival. The topoisomerase I (topo I) inhibitors are a new class of anticancer agents with a wide spectrum of activity in human malignancies. Recent evaluation of the topo I inhibitor topotecan demonstrated activity in advanced myeloma, suggesting a possible role for these drugs in the treatment of this disease. Further evaluation of the mechanisms of resistance to topo I inhibitors, study of combination therapy with topotecan, and evaluation of other topo I poisons in multiple myeloma is proposed.

This review presents a summary of preclinical and clinical data on the topoisomerase I (topo I) inhibitors that are under clinical development. To date, all of the topo I inhibitors that have been clinically evaluated are analogues of camptothecin, an extract of the Chinese tree Camptotheca acuminata. The therapeutic development of camptothecin was initially limited by its poor solubility and unpredictable toxicity. More recently, a number of water-soluble camptothecin analogues have undergone extensive evaluation and have demonstrated significant clinical activity. These include irinotecan (CPT-II), topotecan, and 9-aminocamptothecin (9-AC). Preliminary data are also reviewed on other camptothecin analogues (GG-211 and DX-8951f), on oral formulations, and on non-camptothecin topoisomerase I inhibitors. The topoisomerase I inhibitors have already demonstrated a broad spectrum of antitumour activity, most probably due to their unique mechanism of action and lack of clinical cross-resistance with existing antineoplastic compounds. The challenge for the next five years is to identify ways to integrate the topo I inhibitors into multidrug and multimodality therapies to achieve optimal antitumour effect, while keeping the side effects of these therapies manageable.

The nuclear enzyme DNA topoisomerase II (topo II) is the target of important antitumor agents such as etoposide. Recent work has classified topo II targeting drugs into either topo II poisons that act by stabilizing enzyme-DNA cleavable complexes leading to DNA breaks, or topo II catalytic inhibitors that act at stages in the catalytic cycle of the enzyme where both DNA strands are intact and, therefore, do not cause DNA breaks. Accordingly, catalytic inhibitors are known to abrogate DNA damage and cytotoxicity caused by topo II poisons. In this commentary, we have focused on the possibilities of enabling high-dose therapy with the topo II poison etoposide by protection of normal tissue with catalytic inhibitors, analogous to folinic acid rescue in high-dose methotrexate treatment. Thus, we have demonstrated recently that (+)-1,2-bis(3,5-dioxopiperazinyl-1-yl)propane (ICRF-187) enabled a 3- to 4-fold dose escalation of etoposide in mice. Two high-dose etoposide models are described, namely use of the weak base chloroquine in tumors with acidic extracellular pH and targeting of CNS tumors with protection of normal tissue by the bisdioxopiperazine ICRF-187. In conclusion, high supralethal doses of topo II poisons in combination with catalytic inhibitor protection form a new strategy to improve the antitumor selectivity of etoposide and other topo II poisons. Such an approach may be used to overcome problems with drug resistance and drug penetration.

The anthracycline antibiotics doxorubicin (Adriamycin; DOX) and daunorubicin (DNR) continue to be essential components of first-line chemotherapy in the treatment of a variety of solid and hematopoietic tumors. The overall efficacies of DOX and DNR are, however, impeded by serious dose-limiting toxicities, including cardiotoxicity, and the selection of multiple mechanisms of cellular drug resistance. These limitations have necessitated the development of newer anthracyclines whose structural and functional modifications circumvent these impediments. In this review, we will present recent strategies in anthracycline design and assess their potential therapeutic merits. Current anthracycline design has diverged to target either cytoplasmic or nuclear sites. Nuclear targets have been broadened to include not only topoisomerase II (topo II) inhibition through ternary complex stabilization and catalytic inhibition, but also topoisomerase I (topo I) inhibition and transcriptional inhibition. In contrast, cytoplasmic targeting focuses on anthracycline binding to protein kinase C (PKC) regulatory domain with consequent modulation of activity.