Failure of radiotherapy to reverse progressive pulmonary fibrosis caused by paraquat

Paraquat poisonings: mechanisms of lung toxicity, clinical features, and treatment

Paraquat dichloride (methyl viologen; PQ) is an effective and widely used herbicide that has a proven safety record when appropriately applied to eliminate weeds. However, over the last decades, there have been numerous fatalities, mainly caused by accidental or voluntary ingestion. PQ poisoning is an extremely frustrating condition to manage clinically, due to the elevated morbidity and mortality observed so far and due to the lack of effective treatments to be used in humans. PQ mainly accumulates in the lung (pulmonary concentrations can be 6 to 10 times higher than those in the plasma), where it is retained even when blood levels start to decrease. The pulmonary effects can be explained by the participation of the polyamine transport system abundantly expressed in the membrane of alveolar cells type I, II, and Clara cells. Further downstream at the toxicodynamic level, the main molecular mechanism of PQ toxicity is based on redox cycling and intracellular oxidative stress generation. With this review we aimed to collect and describe the most pertinent and significant findings published in established scientific publications since the discovery of PQ, focusing on the most recent developments related to PQ lung toxicity and their relevance to the treatment of human poisonings. Considerable space is also dedicated to techniques for prognosis prediction, since these could allow development of rigorous clinical protocols that may produce comparable data for the evaluation of proposed therapies.

Effect of lung irradiation on mice following paraquat intoxication

To determine the effect of lung irradiation in mice with paraquat poisoning and whether irradiation could prevent lung destruction, we found the the lethal dose for 50 percent survival (LD50) in 50 mice (Swiss strain) was 25 mg/kg of body weight. The first control group comprised 15 mice and their lungs were irradiated with 200 rad for seven days. There was no death within six weeks. The lungs at autopsy appeared mildly congested. The control group comprised 15 mice. They were injected with 25 mg/kg of paraquat. Six died within three days. The experimental group included 14 mice injected with paraquat and with lung irradiation. Eight died within three days (p greater than 0.5). The pathologic findings in the experimental group revealed lung congestion and fibrosis that was more pronounced than in the group that received only paraquat. We conclude that radiotherapy is not effective in amelioration of paraquat-induced lung damage in mice.

Radiotherapy for the treatment of pulmonary complications of paraquat poisoning

The effect of radiotherapy on the pulmonary damage caused by paraquat (24% solution of 1,1'-dimethyl-4,4'-bipyridylium dichloride) was investigated in a preliminary series of nine patients. Paraquat intoxication was diagnosed by quantitative analysis of urine and plasma using colorimetry after extraction of a cation exchange column. The irradiation was given as a planned procedure from day 2 in cases 1 to 7, and after changes in chest X-ray were recognized in cases 8 and 9. A cobalt-60 unit with opposed anterior and posterior portals was used to give a total dose of 12.50 Gy (uncorrected) over 10 fractions, sparing the pericardium and mediastinum as much as possible. Each fraction consisted of 1.25 Gy (125 rad) given once a day, alternating between the left and right lungs. Radiological diagnosis consisted of clear chest X-rays (cases 2 to 4), pulmonary oedema (cases 7 and 9) and interstitial infiltrates (cases 1, 5, 6 and 8). Cases 1, 5 and 8 survived. Case 8 had residual interstitial infiltrates three months after ingestion but these had cleared one month later, suggesting that the diagnosis of irreversible fibrosis should only be made after a follow-up period of at least one year. The results have failed to show a definite benefit, but do support the fact that radiotherapy should be assessed carefully in a randomized trial which is now in progress.

[Failure of chemotherapy and radiotherapy in pulmonary fibrosis caused by paraquat]

A case is reported of severe interstitial paraquat poisoning in a 23 year old man. Initial treatment consisted of gastric lavage, fuller's earth, haemoperfusion on activated charcoal and peritoneal dialysis. On the third day, appeared both renal and early respiratory failure, with hypocapnia and hypoxia. Peritoneal dialysis was kept up to the thirteenth day. Pulmonary function tests showed a restrictive syndrome. Chemotherapy with 1 mg.kg-1.24 h-1 prednisone and 3 mg.kg-1.24 h-1 cyclophosphamide was started on day 4. Radiotherapy (13 Gy) was also given to both lung fields between days 6 and 10. Despite this treatment, respiratory failure worsened, and the patient died at the 28th day, with resistant severe hypoxaemia and pneumothorax on fibrous lungs. Several reports have shown conflicting results with chemotherapy and radiotherapy in the treatment of pulmonary fibrosis due to paraquat poisoning. The failure of such treatment in the case reported showed the need for large prospective clinical trials of the treatment of paraquat poisoning, as well as for urgent preventive measures.

Paraquat lung: is there a role for radiotherapy?

We have previously reported the clinical course of a patient poisoned with paraquat who was treated with whole lung irradiation and who survived severe pulmonary damage. Four further cases are reported who were much more severely poisoned and who died despite early pulmonary irradiation. There is no definite evidence that this or any other form of cytotoxic therapy can influence the course of paraquat lung. As there is no adequate laboratory model, further investigation of such therapy should be in the context of a prospective clinical trial. Treatment of only one lung would allow rapid identification of a therapeutic effect of irradiation.