At the present time, with the exception of the few neoplasms mentioned in Table 20.1, chemo- therapy is used either when the principal cancer therapies of surgery and radiotherapy have not been effective or in conjunction with one or both of these modalities. We have already noted the effectiveness of combining radiotherapy with chemotherapy in the treatment of Hodgkin dis- ease. Today in many instances, even when the primary surgical or radiotherapies are thought to be reasonably successful, chemotherapy will be used in addition as “adjuvant” therapy. This course of action is seen not infrequently in the treatment of breast, colon, prostate, and a number of other neoplasms more commonly seen in older age groups. Such adjuvant therapy has been extremely useful in extending the life span and even achieving complete cures with breast cancer (Olivetto et al., 1994) and to a lesser extent with colon cancer (Shulman and Schilsky, 1995). Since the adjuvant therapy is designed toward cytotoxic and/or cytostatic effects on any neoplas- tic cells that might remain in the host after surgery and/or radiotherapy, the difficulties with po- tential side effects and subsequent toxic consequences, even to the development of secondary neoplasms, must be considered. Theoretically, the ideal adjuvant therapy for neoplasia after re- moval of the majority of neoplastic cells within the host (debulking) is the use of immunother- apy, which depends to a great extent on the immune system of the host itself augmented with external, immunologically active agents. This subject was discussed more fully in Chapter 19, where it was pointed out that immunotherapy has the best chance of eliminating all neoplastic cells if the host recognizes them as foreign antigens. Thus, the combination of immunotherapy and chemotherapy should prove to be one of the most effective means of treating neoplasms to obtain complete cures. Some human neoplasms, particularly choriocarcinomas in females and Burkitt lymphoma, are highly antigenic and can be consistently cured by chemotherapy. This effectiveness probably occurs through a mechanism whereby the drug kills the majority of neo- plastic cells and the immune response then destroys the remainder of the neoplastic population. A variety of potential mechanisms for such therapeutic activity by the combination of chemother- apy and immunotherapy have been suggested by Talmadge (1992). These are listed in Table 20.11. Note from the table that these mechanisms involve alteration of suppressor cell activity, cytokine and lymphokine enhancement of normal stem cell development, and responses to neoplastic cells, as well as an interesting suggestion that stimulation of tumor cell growth may occur with a subse- quent increase in the sensitivity to chemotherapeutic agents. Kedar and Klein (1992) have also pointed out that chemotherapeutic agents may potentiate immunotherapy by increasing the sensi-
Table 20.11 Potential Mechanism of Therapeutic Activity by Chemoimmunotherapy
1. Reduction in tumor burden by cytoreductive agents resulting in a tumor burden more amenable to immunotherapy
2. Decreased suppressor cell activity resulting in an increased T cell adjuvant activity induced by agents such as r11 IL-2
3. Increased therapeutic activity of two cytoreductive-cytostatic modalities such as a chemotherapeutic agent and tumor necrosis factor or interferon
4. Accelerated myeloid restoration due to increased stem cell activity, thereby allowing more aggressive doses of chemotherapy and reduced neutropenia
5. Addition of agents that stimulate stem cell cycling and accelerate myeloid restoration in conjunction with autologous bone marrow following aggressive cytoreductive therapy
6. Stimulation of tumor cell growth and subsequent increase in sensitivity to chemotherapeutic agents
After Talmadge (1992) with permission of the author and publisher.
tivity of neoplastic cells to immunological attack, imposing antigenic changes on the cells by act- ing as a hapten, and potentiating the stimulation of effector cells as a consequence of massive release of tumor antigens by induced apoptosis. Thus, as we learn more of the mechanisms in- volved in the immunobiological aspects of the host–tumor relationship and develop more effective chemotherapeutic agents, the combination of the two offer perhaps the best hope to ultimately suc- cessfully eliminate the “last remaining neoplastic cell” in the host, which effects an absolute cure.
RECENT MODALITIES IN AND POTENTIAL FOR CANCER CHEMOTHERAPY
Until recent years, the principal direction of cancer chemotherapy has been toward newer and better drugs aimed at affecting cell replication as well as by endocrine-active drugs. A very significant portion of the drugs presently in use were discovered as a result of serendipity or their efficacy is directly related to serendipitous findings. With the dramatic increase in our knowledge of the cellu- lar and molecular biology of living tissues, both normal and neoplastic, it is now reasonable to de- vise chemotherapeutic agents on the basis of several rationales. Several of these are discussed below.
Signal Transduction Pathways as Targets for Chemotherapy
With a dramatic increase in our knowledge of molecular mechanisms involved in signal trans- duction and its aberrations in neoplasia, components of this pathway have been suggested as possible targets for chemotherapy (Powis, 1994). Although there are many possibilities, certain specific sites have been targeted by agents developed specifically for such effects. A diagram of the Ras/MAP kinase cascade is seen in Figure 20.17. In the figure are indicated sites at which drug intervention has been developed and attempted.
Growth Factor Receptor Targets
While a variety of growth factors are involved in the development of neoplasia, predominantly during the stage of promotion but continuing into the stage of progression, reagents—both drugs and natural products—have been developed in an attempt to selectively alter or modulate such functions. The epidermal growth factor family has been one target by use of inhibitors of the tyrosine kinase function of the receptor, which has interesting potential, as well as interfering with ligand-receptor interactions (Davies and Chamberlin, 1996). The latter approach has been relatively successful and is now in clinical trials through the use of specific monoclonal antibod- ies directed toward members of the epidermal growth factor receptor family that are expressed on many highly malignant neoplastic cells (cf. Disis and Cheever, 1997). Investigations have also attempted to utilize modulation of the receptor to alter drug resistance (cf. Davies and Chamberlin, 1996). In addition, in a recent study (Wosikowski et al., 1997) with cell lines and computer analysis of cytotoxicity patterns, several compounds were identified as inhibitors of the epidermal growth factor receptor pathway, probably by modulating phosphorylation. Some studies have also demonstrated that both natural and synthetic agents may modulate the expres- sion of growth factors on neoplastic cells (Tagliaferri et al., 1994).
Protein Kinase Inhibitors
As noted above, some disruption of the epidermal growth factor pathway may be effected by inhibiting the tyrosine kinase component of the receptor. A large number of other protein kinase inhibitors have also been synthesized and utilized, primarily in in vitro systems (cf. Boutin,1994). A variety of structures of such inhibitors can be seen in Figure 20.18. Some of these are
Figure 20.17 Signal transduction pathways by the Ras/MAP kinase cascade as well as via protein kinase C and subsequent activation of the nuclear transcription factor NFκB. The student is referred to ear- lier figures (7.6, 7.7, 7.8) giving some details of the specific pathways. Inhibitory actions of drugs and natural products are noted by the numbers within circles adjacent to the reaction and its product. (1) Growth factor-receptor inhibition; (2) inhibition of protein kinases; (3) inhibition of prenylation of G pro-teins. (Modified from Hinterding et al., 1998, with permission of the authors and publisher.)
relatively selective for the epidermal growth factor receptor such as DAPH-1 (cf. Patrick and Heimbrook, 1996). Inhibitors of the protein kinase C family of serine/threonine protein kinases have involved both the use of active-site-directed inhibitors targeting both peptide substrate and nucleotide binding sites. However, cell-penetration problems remain as important factors in the use of these agents. Nucleotide binding-site-directed inhibitors include H-7 and staurosporine, while R0 32-0432 is an ATP-competitive inhibitor of some members of the protein kinase C family (cf. Patrick and Heimbrook, 1996). Inhibitors of the raf serine/threonine protein kinase and the mitogen activated kinases (MEK, MAP) have also been described affecting several mem-
Figure 20.18 Structural features of some protein kinase inhibitors. The reader is referred to the original reference (Patrick and Heimbrook, 1996) and the text for further details. (Adapted from Patrick and Heim- brook, 1996, with permission of the authors and publisher.)
bers of this pathway. The example from Figure 20.18 is PD 98059, which inhibits both the MEK- kinase or raf-dependent activation of MEK but does not seem to affect raf autophosphorylation (cf. Patrick and Heimbrook, 1996). In addition to inhibitors of signal transduction kinases, there are also being developed chemotherapeutic agents capable of altering the function of cyclin-dependent kinase inhibitors that may have important usefulness in chemotherapy (Kaubisch and Schwartz,
2000). Despite the extensive work in this area and the large involvement of protein kinase activities in signal transduction pathway, one must remember that all of these pathways function in normal cells, and thus the differential between effective treatment in neoplasia and lack of toxicity in nor- mal tissues (therapeutic index, see above) may be less than desired in many instances.
Protein Prenylation as a Target for Chemotherapy
As noted in Figure 20.17, the ras protein is shown bound to the plasma membrane through a lipid moiety, farnesyl, which is covalently linked to a cysteine sulfhydryl of the protein, forming a thioether bond. As briefly discussed in Chapter 7, this interaction of these G proteins with the cell membrane is required for normal signal transduction. Inhibitors of the formation of isoprenoids, especially farnesyl and geranyl moieties and/or their linkage to the protein, can result in inhibi- tion of signal transduction pathways. This has been demonstrated by the rational design of pepti- domimetics of the carboxyl terminal tetrapeptide farnesylation site on ras (Lerner et al., 1997; Moasser et al., 1998). The resulting agents are capable of inhibiting ras processing, selectively antagonizing oncogenic signaling, and suppressing neoplastic growth in mouse models with rela- tively small side effects. While there are a number of key problems that still must be overcome in order for such agents to enter clinical trials, other inhibitors of isoprenoid synthesis may become effective additions to the chemotherapeutic armamentarium (Waddick and Uckun, 1998).
On the other hand, the variety of signal transduction pathways extending all the way into nuclear factors and mechanisms may be amenable to attack by the use of structure-based strate- gies by drug design and discovery (Kuntz, 1992). The modern technologies involving combinato- rial chemistry as well as detailed structural knowledge of the protein targets, especially their active sites, can lead to the development of drugs extremely specific in their site of action with a much greater potential for an effective therapeutic index (Lam, 1997). One potential target for such strat- egies would be the nuclear factor κB, which affects cell survival and determines the sensitivity of neoplastic cells to cytotoxic agents as well as to ionizing radiation (Waddick and Uckun, 1999). In Figure 20.17, this pathway may be noted as being related to a cytoplasmic factor, IκB, which nor- mally maintains NFκB as a complex but when phosphorylated by protein kinase C releases the active NFκB to become involved in specific gene transcription within the nucleus. A recent exam- ple of the use of such technologies is the synthesis of monastrol, an agent that specifically inhibits the motility of a mitotic motor protein required for normal spindle structure (Mayer et al., 1999). It is very likely that, as more basic knowledge of the structural characteristics of specific proteins within signal transduction pathways become known, specific drugs can be exquisitely tailored to interact and alter the effectiveness of these gene products within neoplastic cells.