Experimental design considerations in clinically combining radiation and drugs

From targets to genes: a brief history of radiosensitivity

The biological work of Douglas Lea spanned the period from 1934 to his early death in 1947, and during this short period he made important contributions to the theory of radiation action. He interpreted experimental data relating to the effects of radiation on viruses, bacteria, bean roots, etc in terms of the inactivation of discrete targets, which he identified with cellular genes. He thus laid the foundation of much subsequent research. It is now well recognized that mammalian cells differ substantially in radiosensitivity, especially in the low-dose region of the survival curve. The dependence of radiosensitivity on dose rate has been widely studied; this has practical significance for clinical radiotherapy as well as mechanistic implications. Since Lea's time there have been a number of efforts to describe models that can relate cell killing to radiation dose, dose rate, and track structure. So far these have not led to a comprehensive and widely accepted picture. Microdosimetric considerations lead to the concept of differing severity of lesions induced in DNA. Much is known about the sequence of processes that subsequently lead to cell inactivation: this can be divided into phases of induction, processing, and manifestation. Chromosomal events are currently attracting much attention, as they did in Lea's time. Considerable progress has also been made in identifying genes that control the repair of radiation damage. It has been found that mutation is frequently associated with the loss of a large segment of the genome around the damage site and this will have important implications for interactive processes between particle tracks.

Treatment of the rat R-1 rhabdomyosarcoma with methotrexate and radiation; effects of timing on cell survival and tumour growth delay

The interaction of radiation (10 Gy 300-kV X-rays) and methotrexate (MTX; 3 x 10 mg/kg at 3.5-h intervals) was investigated with respect to effects on cell survival and tumour regrowth of the transplantable rat R-1 rhabdomyosarcoma. The treatment with MTX alone caused acceptable toxicity and no lethality. On day 3 after treatment with MTX alone a maximum decrease in the fraction of clonogenic cells was observed, which is in accordance with data on MTX concentrations in tumour tissue, indicating that MTX is active in the tumour for at least 3 days after injection. The clonogenic capacity after combined treatments, i.e. MTX before or after radiation, was assessed 3 days after the MTX administration. The fractions of clonogenic cells determined after combined therapy with intervals of up to 4 days were not significantly different from those expected on the basis of simple addition of the effects from individual treatments. However, the excess growth delay was positive at specific intervals (6-8 days after X-rays plus MTX and 5-6 days after MTX plus X-rays), whereas negative excess delays were observed when the two treatments were separated by less than 3 days. It is concluded that expectations with respect to clinical application of the combination must be modest in view of the short duration of favourable intervals and the observed absence of synergistic effects with respect to cell killing. The discrepancy between the two assays indicates that erroneous conclusions can be obtained if one endpoint only is assessed.

Biological basis for the interaction of chemotherapeutic agents and radiation therapy

The ultimate goal of combining chemotherapy and radiotherapy is to improve the therapeutic ratio. When chemotherapy is combined with radiotherapy, improved therapeutic effects may occur as a result of the following: (1) spatial cooperation; (2) enhancement of tumor response with less or no enhancement of normal tissue injury, and (3) diminution of normal tissue injury with less or no diminution of tumor response. Possible mechanisms of interaction between chemotherapeutic agents and radiation that result in an enhanced effect include (1) changes in the slope of the dose response curve, (2) decreased accumulation or inhibition of repair of sublethal damage, (3) decreased recovery from potentially lethal damage, (4) perturbation in cell kinetics with an increase of proportion of cells in the sensitive cell cycle phase and proliferative state, (5) decreased tumor bulk and improved blood supply leading to reoxygenation and recruitment and increased radiosensitivity and chemosensitivity, and (6) increased drug delivery and uptake. The combined effects may be influenced by (1) tumor and normal tissue type, (2) drug type, (3) drug dose and schedule, (4) time sequence between drug and radiation administration, (5) radiation dose and fractionation schedule, and (6) radiation dose rate. Most experimental and clinical data suggest that enhanced tumor effects most often result from simple additivity and therefore do not require direct interaction between drug and radiation, whereas enhanced normal tissue effects are observed most often when drugs are administered in close temporal proximity to radiation. Thus, for the optimal therapeutic effect, it would seem more advantageous to administer chemotherapeutic drugs and radiation in a sequential or alternating manner rather than simultaneously.

Sequencing of drugs and radiation. The integrated alternating regimen

The association of radiotherapy (RT) and chemotherapy (CT) constitutes one of the main avenues for research in therapeutic oncology. This association has two aims: (1) increase in control rate of primary tumor (this requires either the potentiation of one of the two modalities by the other or the additivity of their effect on tumor cells without a parallel increase in the toxic effects on critical normal tissues); (2) spatial cooperation (RT being used for the control of the primary tumor or of the sanctuaries, and CT for the control of the disseminated disease). In these two strategies, RT and CT should be given up to full doses in order to be effective. The main risk is an increase in the number and severity of the early and late side effects. To circumvent this problem, two possibilities are being explored: (1) use of drugs without serious toxic effects on those critical tissues which are included in the irradiated volume; and (2) avoidance of concomitant administration and introduction of a sufficiently long-time gap between the completion of one modality and initiation of the other. However, in such sequential treatment, a delay of CT until after the completion of RT, or an interruption of CT cycle during the course of RT, allows the occult metastases to increase in size; a similar delay in initiation of RT is also detrimental, as drugs are often not effective on bulky tumors. Moreover, under CT, the cells which are resistant to the cytotoxic drugs may disseminate and initiate chemoresistant metastases. Taking these disadvantages into account, a treatment protocol was proposed in 1980 in which CT and RT are given alternately, without undue delay. Chemotherapy is started with the usual scheduling of one cycle every month. Radiotherapy courses are interdigitated between CT cycles. Each course is initiated 1 week after interrupting CT and continued until 1 week before beginning the subsequent cycle of chemotherapy, and so on until completion of RT. Such split-course RT should have an effect on a tumor comparable to that of a conventional fractionation. This protocol has been used on 24 patients with non-Hodgkin's lymphoma (NHL) of diffuse histology, and 63 patients with small cell carcinoma of the lung. The 2-year relapse-free survivals are promising (in clinical stage II NHL of diffuse histology, 75%; and in small cell lung carcinomas, 33%).(ABSTRACT TRUNCATED AT 400 WORDS)

Treatment of carcinoma of the head and neck with combined irradiation and chemotherapy

Radiation therapy was combined with simultaneous administration of bleomycin, cyclophosphamide and 5-fluorouracil for the treatment of 33 patients with carcinoma of the head and neck. This combination increased the toxicity to the normal tissue. Complete response was obtained in 46 per cent, partial response in 36 per cent and no response in 18 per cent. Patients with a complete response had a significantly longer survival than patients with a partial or no response. An increased cytotoxicity was noted after the combined treatment when compared with irradiation alone. However, no evidence of an increased therapeutic effect was obtained with the addition of simultaneous chemotherapy to irradiation.