BASIC PRINCIPLES

Chemotherapy drugs are developed for their potential to cause a greater proportion of cell death among neoplastic as opposed to normal cells. Differences exist between nor­mal and malignant cells that result in the latter being more susceptible to anti-cancer drugs by virtue of their bio­logical and proliferation characteristics.

Cancer cell kinetics

CHARACTERISTICS OF THE TUMOUR CELL

The proliferation of tumour cells is not entirely autonomous and there is increasing evidence of local control by autocrine and paracrine factors produced by the tumour cells and the stroma. The rate of proliferation during the lifetime of a tumour is not constant. In experimental tumours in the early stages, growth is exponential and the growth fraction ’s high. As the tumour enlarges, the growth rate slows and the growth fraction falls. A decreased rate of growth is commonly observed for trans plan table tumours in animals and probably results, in part, from decreasing tumour vas-cularity and cellular nutrition leading to slowing of cell proliferation, and also from cell loss due to death or differ­entiation. The smallest tumour that is likely to be clinically detectable (either by physical or radiological assessments) will be approximately 1 cm in diameter and will contain 10⁸-10⁹ tumour cells, depending on the contribution of stroma and other elements to tumour bulk; growth of these small, clinically detectable tumours follows a Gompertzian pattern. Such a tumour will have undergone approxi­mately 30 doublings in cell number if it is clonally derived from a single transformed cell, and will usually weigh about 1 g. Growth to a potentially lethal mass of 1 kg of tumour requires only a further 10 doublings of cell num­ber. Thus the period of tumour growth that is clinically apparent is only a relatively short period in the total life history of a tumour, and clearly the potential exists for micro-metastases to develop before detection of the pri­mary tumour.

Worth to visit:

• life insurance rates
• alternative cancer treatment

The effect of chemotherapy on a tumour is influenced by some of the features of its growth pattern:

• Response   to   chemotherapy   is   proportional   to   the number of cells synthesizing DNA.
• The shorter the doubling time at the onset of treatment (i.e. the more rapid is tumour growth), the better the response to chemotherapy, since more cells will be synthesizing DNA.
• As the tumour grows, the disease becomes less easy;

• As the tumour shrinks with treatment, the growth rate increases because of the Gompertzian growth pattern.

CELL CYCLE

All proliferating cells go through a series of events that comprise the cell cycle. The division of the cell cycle into

discrete phases followed the demonstration that DNA syn thesis took place during a defined time interval, rather than continuously during interphase. After mitosis (M), the cell spends a variable resting period (Gl) during which DNA synthesis does not occur but RNA and protein are pro­duced. Entry to the S phase is heralded by an increase in RNA synthesis followed by doubling of the DNA content. The G2 phase follows as DNA synthesis ceases, and is fol­lowed by mitosis.

The total duration of the cell cycle depends mainly on the duration of the Gl phase, which may be 0-30 hours. The durations of the S phase (6-8 hours), M phase (less than 1 hour) and G2 phase (2-4 hours) are fairly constant both in normal and malignant cells. Consequently, the cycle of a malignant cell may last between 9 and 43 hours. For aggressive, highly proliferative tumours, the cell cycle will be at the shorter end of the time span, but for more indolent, low-grade tumours, the cell cycle will be signifi­cantly longer. The mean cycle time of cells within human tumours is typically much shorter than the mean volume-doubling time of the tumours for two main reasons: a high rate of cell death, and a high proportion of non-proliferating cells. The term GO is applied to cells that are out of cycle. The proportion of cells within a population that is under­going active proliferation in the cycle is termed the growth fraction. Estimates of growth fraction calculated by com­paring the measured proportion of cells in S phase with that predicted from the phase distribution of cycling cells are consistently of the order of 20-30 per cent. This is par­ticularly relevant, as most anti-cancer drugs do not cause cell death during the GO phase. Furthermore, although higher proportions of S-phase cells are found in some rap­idly growing tumours such as high-grade lymphomas, most tumours do not have a higher proportion of S-phase cells than some normal highly proliferative tissue such as bone marrow and intestinal crypt cells.

Mathematical models have been developed to describe the interaction of cytotoxic chemotherapy and tumour growth kinetics and may be used to evaluate hypothetical strategies for cancer treatment.

ANTI-CANCER DRUGS AND THE CELL CYCLE

Cytotoxic chemotherapy agents have traditionally been classified as phase or non-phase specific, depending on the effect on the cell cycle (Table 5.1). In-vitro models demon­strate that phase-dependent drugs kill cells exponentially at lower doses but reach a plateau when given at a higher dose because they are only able to kill cells in a specific part of the cell cycle. Non-phase-dependent drugs kill cells exponentially with increasing dose and are equally toxic both for cycling cells and those in GO. The practical value of this classification is somewhat limited in that chemotherapy regimens designed on kinetic prin­ciples have so far shown no advantage over those derived empirically.

Table 5.1 Cytotoxic drugs and the cell cycle

Predominantly non-phase-specific agents	Predominantly phase-specific agents
Nitrogen mustards Cyclophosphamide Melphalan Chlorambucil Busulphan Thiotepa 5-Fluorouracil Doxorubicin Mitomycin-C Dacarbazine Actinomycin-D	Methotrexate Cytosine arabinoside 6-Mercaptopurine 6-Thioguanine Vincristine Vinblastine Bleomycin Etoposide Procarbazine

SKIPPER HYPOTHESIS FOR CELL KILL BY CYTOTOXIC AGENTS

In the early 1960s, Skipper et al. formulated some princi­ples of tumour cell kill on the basis of experiments using the L1210 murine leukaemia model:

•   The survival of an animal is inversely related to the

tumour burden.

•   A single leukaemic cell is capable of multiplying to kill the host.

•   For most drugs there is a clear relationship between dose of drug and eradication of tumour cells.

•   Cell destruction by anti-cancer drugs follows log kill kinetics. That is, a given dose of drug kills a constant fraction of cells and not a constant number. Thus if a particular dose of an individual drug kills 3 logs of cells and reduces the tumour burden from 10¹⁰ to 10′ cells, the same dose used at a tumour burden of 10³ will reduce the tumour burden to 10² cells. The cell kill is therefore proportional regardless of tumour burden.

These principles established that there is an inverse rela­tionship between cell number and curability and imply that tumours are best treated when they are small in vol­ume. Furthermore, if drug treatment is discontinued as soon as the tumour is no longer clinically detectable, at least 10⁹ tumour cells remain and relapse is inevitable. However, these observations should be considered in the context of the growth differences between this murine leukaemia model and human cancers. For example, L1210 leukaemia is a rapidly growing tumour, with a high per­centage of cells in S phase and a growth fraction of 100 per cent, giving a consistent and predictable cell cycle. In con­trast, the cell cycles of human tumours are heterogeneous, prolonged and with a smaller growth fraction.

Overall, the evidence favours a Gompertzian growth pat­tern with a growth fraction that is not constant and with an exponential decrease in growth rate as the tumour enlarges.

Nevertheless, Gompertzian kinetics also support the notion that chemotherapy is more likely to be effective in eradicating a small tumour burden. When the tumour bur­den is small (such as when no longer clinically detectable), its growth fraction would be at its largest, and the propor­tional cell kill would be larger. This is one of the principles that form the basis of adjuvant chemotherapy strategies.
Related resource: generic viagra

NORTON-SIMON HYPOTHESIS

In tumours that show Gompertzian-type growth curves, ,he rate of re-growth increases as the tumour shrinks with therapy. Thus the level of treatment necessary to initiate a regression may be insufficient to maintain the regression and produce cure. Norton and Simon⁵ hypothesized that _t0 overcome the slowing rate of regression in a tumour responding to therapy it was necessary to increase the intensity of treatment as the tumour became smaller. This can be achieved in one of two ways:

1) increase the dose intensity of the chemotherapy agents used to induce remission;

2) switch to alternative cytotoxic agents in an aggressive schedule.

Dose intensity is commonly used in leukaemia, for which agents such as cytosine arabinoside are used in high-dose pulses after the induction of remission. In addition, high-dose chemotherapy with bone-marrow transplant­ation or peripheral blood stem-cell harvesting with growth factor support is another means of achieving dose inten­sity. The concept of dose intensity is discussed more fully later in this chapter.

Alternatively, the use of other cytotoxic agents in hybrid regimens such as MOPP-ABVD for Hodgkin’s disease⁴ not only exposes the tumour to drugs that are different from those used to achieve induction or remission, but also attacks residual populations of cells that are biochemically resistant to the initial combination of drugs.

GOLDIE-COLDMAN MODEL

Spontaneous mutation is a basic property of DNA. There is also evidence that tumour cells may be more genetically unstable than normal cells. In 1979, Goldie and Coldman proposed a model to explain the genetically determined resistance to cancer chemotherapy based on this principle. They proposed that populations of cells within a tumour are capable of randomly mutating and becoming resistant to the cytotoxic agents. These spontaneous mutations occur at population sizes of less than 10⁶ tumour cells, which is less than the clinically detectable level. As such mutations occur at frequencies of 10^-6 or higher, a clini­cally detectable tumour of 10⁹ cells is likely to have several drug-resistant clones. However, the absolute number of resistant cells would be relatively small and these tumours would probably respond  initially to treatment with  a complete or partial remission, only to relapse and re­appear when the resistant clone(s) expand, a clinical pic­ture that is familiar in oncology practice. Some tumours are resistant to cytotoxic chemotherapy agents even when they present with a relatively small tumour volume. However, such slow-growing tumours may have consider­able cell loss through cell death – up to 90 per cent of the tumour volume. Therefore what appears to be an early tumour may have gone through many more cell doublings than expected in order to compensate for cell loss in achieving that size. Consequently these cells may have undergone a higher frequency of spontaneous mutations, leading to a tumour that consists predominantly of drug-resistant clones.

However, there are many other mechanisms of drug resistance, including decreased uptake due to changes in drug-specific transport mechanisms, decreased activation of pro-drugs, alterations in cellular metabolism and repair mechanisms, increased inactivation of drugs, target alter­ations and acquisition of a multi-drug-resistance pheno-type. Drug resistance is discussed more fully in a later section of this chapter.

Different Resources: