Fd Comer.
Toxicol.
Vol. 14. pp. 653-654.
Pcrgamon
Press 1976. Printed
in Great Britain
LETTERS TO THE EDITOR PHARMACOKINETICS:
FIRST
ORDER
OR ZERO
ORDER?
Sir,-1 have read the paper by Hiles & Bruce (Fd Cosmet. Toxicol., this issue,p. 599) and agreein part with their objection. Indeed, their objection was the primary reasonwhy a second,much more definitive, study was condkted on the fate of 2,3,7,8-tetrachlorodibenzo-p-dioxin following single and repeatedoral dosesto the rat (Roseet al. Toxic. appl. Pharmac. 1976,36. 209).I am sure that if they review this article Drs Hiles and Bruce will concedetheir objection.First, a much lower singleoral doseresultedin the same kinetics,attesting to a first-order process.Even more important is the fact that repeateddaily dosesover a lOO-folddaily doseregimenprovided very sound evidencefor a first-order process.On the basisof the latter study, I feel that Hiles & Bruce (lot. cit.) have selecteda poor example to illustrate a point that is generallywell known. The more recent study, however, raisesthe questionof why we publishedthe earlier one knowing full well that our assumptionof first-order elimination might be wrong. First, we were limited in technology at the time, becauseno 14C-labelled 2,3,7,8-tetrachlorodibenzo-pdioxinwasavailablewith a specificactivity higher than 2mCi/mmol, and this precludedadministrationof smallerdoses.In addition to the useof a dosethat causedtoxicity, retainingsickrats in a metabolismchamberfor morethan 21 days wasunreasonable, so it wasnot possibleto follow the treated rats through four or more half-lives. Secondly,although in our judgementa first-order processwas the more likely, we presentedthose data, statedthat a first-order processwas assumedand defined the rate constantas an apparent first-order rate constant. In this way we left the matter open for other interpretation. The more recent study, of course, substantiatesour original judgement. Finally, but mostimportantly, our original assumptionof a first-order rate processwasthe more conservative, insofar as safety assessment was concerned.Assuminga zero-order processwould have indicated, in essence, a saturableprocessand hencea greater eliminationrate as the dosewas decreasedor as the body burden decreased.Unfortunately, our recent study did not prove this to be true. When in doubt, I feel justified in selectingthe conservativesidewhen a questionof safetyassessment is in the balance. In conclusion,I accept the criticism of Hiles & Bruce (lot. cit.). Since much of our work is concerned with saturablemetabolismor non-linearpharmacokinetics,I would, indeed,find it hard not to accept.However, as I have indicated, they unknowingly selecteda very poor example; many other more appropriate examplesexist. I encouragetheir enthusiasm and hopethat it leadsto somestudieson saturableor non-linear pharmacokineticsof chemicals.We are certainly in dire needof more examplesin the literature by qualified investigators. P. J. GEHRING, Toxicology Research Laboratory, Health and Environmental Research, 1803 Building, Dow Chemical USA, Midland, Mich. 48640, USA
DIETARY
NITROSAMINES
Sir,-To make the excellent review article on dietary nitrosamines(Cooper, Fd Cosmet. Toxicol. 1976,14, 205)more complete,we would like to draw your attention to the following additional data. When 149 samplesof fish mealswere analysedby Kowalski & Cybulski (Bull. vet. Inst. Pulawy 1974, 18, 96), 51 (34%)of the samplescontainednitrates in concentrationsof l-300ppm (mg/kg), and 10 (67%) containednitrites in concentrationsof 1-12ppm. Of 16 positive samplesselectedfrom thosewith the highest concentrationsof nitrates and nitrites, 14 contained dimethylnitrosamine(DMNA) in concentrationsof [email protected], while two samplesalso containeddiethylnitrosamine(DENA) in concentrationsof 0.028 and 0036ppm. It has beenreported (Juszkiewicz& Kowalski, in N-Nitroso Compounds in the Environment, edited by P. Bogovski and E. A., Walker; IARC Scient. Publ. no. 9 (1975),p. 173) that the oral administration of singledosesof DMNA, DENA and dipropylnitrosamine(DPNA) to lactating goatsresultedin the excretion of detectableamountsof thesecompoundsin the milk for approximately 24-36 hours, dependingon the doseand the chemicalform of the nitrosamine.Milk concentrationsof 9.7, 4.9 and 5.8ppm, respectively, 653
Toxicol.
Vol. 14. pp. 653-654.
Pcrgamon
Press 1976. Printed
in Great Britain
LETTERS TO THE EDITOR PHARMACOKINETICS:
FIRST
ORDER
OR ZERO
ORDER?
Sir,-1 have read the paper by Hiles & Bruce (Fd Cosmet. Toxicol., this issue,p. 599) and agreein part with their objection. Indeed, their objection was the primary reasonwhy a second,much more definitive, study was condkted on the fate of 2,3,7,8-tetrachlorodibenzo-p-dioxin following single and repeatedoral dosesto the rat (Roseet al. Toxic. appl. Pharmac. 1976,36. 209).I am sure that if they review this article Drs Hiles and Bruce will concedetheir objection.First, a much lower singleoral doseresultedin the same kinetics,attesting to a first-order process.Even more important is the fact that repeateddaily dosesover a lOO-folddaily doseregimenprovided very sound evidencefor a first-order process.On the basisof the latter study, I feel that Hiles & Bruce (lot. cit.) have selecteda poor example to illustrate a point that is generallywell known. The more recent study, however, raisesthe questionof why we publishedthe earlier one knowing full well that our assumptionof first-order elimination might be wrong. First, we were limited in technology at the time, becauseno 14C-labelled 2,3,7,8-tetrachlorodibenzo-pdioxinwasavailablewith a specificactivity higher than 2mCi/mmol, and this precludedadministrationof smallerdoses.In addition to the useof a dosethat causedtoxicity, retainingsickrats in a metabolismchamberfor morethan 21 days wasunreasonable, so it wasnot possibleto follow the treated rats through four or more half-lives. Secondly,although in our judgementa first-order processwas the more likely, we presentedthose data, statedthat a first-order processwas assumedand defined the rate constantas an apparent first-order rate constant. In this way we left the matter open for other interpretation. The more recent study, of course, substantiatesour original judgement. Finally, but mostimportantly, our original assumptionof a first-order rate processwasthe more conservative, insofar as safety assessment was concerned.Assuminga zero-order processwould have indicated, in essence, a saturableprocessand hencea greater eliminationrate as the dosewas decreasedor as the body burden decreased.Unfortunately, our recent study did not prove this to be true. When in doubt, I feel justified in selectingthe conservativesidewhen a questionof safetyassessment is in the balance. In conclusion,I accept the criticism of Hiles & Bruce (lot. cit.). Since much of our work is concerned with saturablemetabolismor non-linearpharmacokinetics,I would, indeed,find it hard not to accept.However, as I have indicated, they unknowingly selecteda very poor example; many other more appropriate examplesexist. I encouragetheir enthusiasm and hopethat it leadsto somestudieson saturableor non-linear pharmacokineticsof chemicals.We are certainly in dire needof more examplesin the literature by qualified investigators. P. J. GEHRING, Toxicology Research Laboratory, Health and Environmental Research, 1803 Building, Dow Chemical USA, Midland, Mich. 48640, USA
DIETARY
NITROSAMINES
Sir,-To make the excellent review article on dietary nitrosamines(Cooper, Fd Cosmet. Toxicol. 1976,14, 205)more complete,we would like to draw your attention to the following additional data. When 149 samplesof fish mealswere analysedby Kowalski & Cybulski (Bull. vet. Inst. Pulawy 1974, 18, 96), 51 (34%)of the samplescontainednitrates in concentrationsof l-300ppm (mg/kg), and 10 (67%) containednitrites in concentrationsof 1-12ppm. Of 16 positive samplesselectedfrom thosewith the highest concentrationsof nitrates and nitrites, 14 contained dimethylnitrosamine(DMNA) in concentrationsof [email protected], while two samplesalso containeddiethylnitrosamine(DENA) in concentrationsof 0.028 and 0036ppm. It has beenreported (Juszkiewicz& Kowalski, in N-Nitroso Compounds in the Environment, edited by P. Bogovski and E. A., Walker; IARC Scient. Publ. no. 9 (1975),p. 173) that the oral administration of singledosesof DMNA, DENA and dipropylnitrosamine(DPNA) to lactating goatsresultedin the excretion of detectableamountsof thesecompoundsin the milk for approximately 24-36 hours, dependingon the doseand the chemicalform of the nitrosamine.Milk concentrationsof 9.7, 4.9 and 5.8ppm, respectively, 653
