Document VJD8VEdYx3D79G5MnGx6JmvDj

nf I ,JI . l ire *atin*ci i'"'n of "V'lOX - rnrch V*. IHM*. rt Mr- 10 nil * Jour- Mi*w vkuIs- trrrnrr, versity phomas xilica. South A*- . MJ- Some etiological considerations of fibre carcinogenesis M F STANTON 1 rtif exogenous .ipents which contribute to the ..nisi- nfc.imer eerier,illv f.ill into one of three major i""i:ps umiMriy r.idt.ttrnn, chemicals and viruses, f here iv .1 wealth of speculation as to how the mem bers of these groups act to induce cancer, but the mechanisms of their action remain unknown. A .hrstos, m all us forms, contains chemicals that aie carcinogenic under certain conditions, however, it is of particular interest as a carcinogen because it has attributes of two of the above groups. Firstly, the presence of various metallic ions and polycyclic hsdrocarhons. that are either inherent or acquired through processing, would seem the best explanation for the carcinogentcttv of asbestos. On the other hand, particles that arc within the dimensional range of viruses arc abundant in all forms of asbestos; and it is conceivable that these submicroscopie particles could act m a fashion similar to that of viruses, whatever that may be. On the other hand, there is reasonably good evi dence that netthe* of these attributes is related to the carcinogrniciiv of asbestos. The evidence for this conclusion can be summarised ax follows: f I > There is no indication that any of the ashesloses are sufficiently contaminated with known carcinogenic hydrocarbons to account for their carcinogenicity; and rigorous extraction of those hydrocarbons present m asbestos does not affect its carcinogenicity for the pleura of the rat (Wagner rt al . 1970). (2) Variations in the inherent metal content of various types of asbestos are great, yet these various tv pc- of asbestos show only slight differences in car cinogenicity (Harington, 1965; Timbrel!. 1970; Vijgnor, 1970; Stanton A Wrench. 1972). ' The t ahofitorv of Pnholofy, RriWi, l`SV C*nc*f Imtiiuic, Hi I mflv p.irnculati* metallic nickel, stainless steel or non-crystalime silicon dioxide applied in the pleura of the ret are not sufficiently carcinogenic to account for the carcinogenicity of asbestos by mill com immation (Stanton A Wrench. 1972). (4) Reduction of fibre si/e by the partial pulverisa tion ofasbestos, a process which increases contamina tion by metallic particles and increases the number of submicroscopie fibrils in asbestos, reduces its car cinogenicity (Stanton A Wrench, 1972). (5) Hand-cobbed crocidolite ore. hand-milled without metallic contamination, is equal in carcino genicity to machinc-milhd crocidolite (Stanton A French, 1972). (6) Non-asbestiform fibres such as fibrous glass are increasingly carcinogenic as they approach (he si/e range of milled asbestos fibre* (Stanton A Wrench. 1972). One must therefore consider that it ts the structural features of asbestos that may be the critical factor in its carcinogenicity, and it is toward this hypothesis that we have directed our attention, if the structural features of asbestos are important, then il follows that similar fibres, if sufficiently durable, should also induce tumours. On the basis of this reasoning, we have assessed the distribution of particles by size in a variety of fibrous materials and applied these to the pleura of rats for a period of two years. These ex periments are still in progress; only preliminary re sults of part of them are available and interpretations are limited. However, estimates of final tumour in cidences have been made from the data currently at hand, and the various materials have been segregated into four groups which cause high, moderate, low and negligible tumour incidence (Tables I -4) The analysis of fibre distribution by size is subject to some error, nevertheless it is sufficient to characterise the general distribution of fibres in the materials. - 2*9 - . ST009577I ST0095772 F,g 1 Graphic iiiwitratio" r*# sent m*in /Jnnens*t-ns ( ^ -*m) of hr* u*+<1 to classify p*ici*s ia th lest samples lHustrifd pa*''* repre MA riRUI S AM) MFTffnns Various specimens of cnvidolife, chrysotile, fihrm. . glass, anil fibrous aluminium oxide were applied In open thoracotomy in the lefi pleural sur face of 10, II- to M-wcek-old. female OsbarneMcndel rats at a uncle standard 40 mg dose level by a method previously described (Stanton era/., 1969). The unique aspecl of these experiment, is that all test materials were applied to small 45 mg fibrous glass pledgets prior to application. The glass pledgets are composed of large-diameter fibrous glass which, when intact, has no apparent carcinogenicity m itself. We use it simply as a convenient and accurate means of uniformly applying the lest material to a wide sur face area of the pleura. The test materials are listed in Tables I -4. The IJICC standard reference samples of crocidolue and chrysolite A have been previously described (Timbrell. 1970). Reduction of panicle size was accomplished by grinding in a stainless-steel Table I High mi.mane* gioups f *0% mesothelioma*) Percantaga ol ma occupied by libm* m earl- rng o( sire indicate extern o< pleural i.tvoen moet commonly obaaned < .ire fuch, t^lf. * tn% -Mrrf nuly rtc*e iteef \ 'V STI*0 fTl'lt Mr.lf Al COSMtlfHATIOV^ n nHf fAKft^nr.rvf.M'i Nil! mnl to protfuce partially or fulls pulverised are shown the five specimens which produced tumour s.implcs. The crude fibres were stripped by hand incidences greater than 40*,, These are the L'lCC from hand-cobbed ore specimens, and bumlles of standard reference samples of crocidolite and chryso- 'iKres rnrr riM,unci! as lone as possible without cor- lile A, two samples of very fine fibrous glass with t. mii'Miiop bs rstr.mroiis mineral diameters of 3 um or less, ansi the aluminium oxide flic fibrous pi.isses were ohi.imed from both the whiskers All of these samples arc composed almost ' *wens-< ornme I iHerglas ( orporatinn. Toledo, entirely of fibres, and further have in common a pre < tfiio and the Johns. Nfitnxillc Research and I ngtnecr- dominance of fibres below ? am in diameter. The u'g < enter. Mansdlc, New Jersey We arc pameu- Al,0, fibres are of particular interest because they *.irlv indebted to the latter institution for the size are totally different from those of asbestos and glass, separatum of fibrous glasses, which was carried out both in internal structure and in chemical composi through a senes of millings and the sedimentation of tion; yet their size distribution is remarkably like secptionalls fine-slumctcr glass fibres. All of the that of L'lCC crocidoliic. However, one-third of the glasses were of the usual borosdicatc type, whose fibres are slightly longer and thicker than are the nnueral oxide contents have been previously re cmcidolite fibres and, since the density of AljO. is corded (Stanton V Wrench. f')72). greater than asbestos, approximately one-sixth as I lie rton-fihrous aluminium oxide and aluminium many fibrous particles are present The Al/), fibres oxide whiskers were commercial products obtained are most durable and do not fragment into sub- from the Arteeh Corporation, falls Church, Virginia, microscopic particles in ihc manner of ashestos these are single crystal fibres that are more than Nevertheless, electron microscope study reveals an '*** v , pure Al/) The method of counting fibres abundance of submicroscopicfihrilsof less thanO. 1 am has been described previously (Stanton A Wrench. in diameter with lengths comparable to those of iv72i. fibres in the lowest optical range. Samples of the materials, suspended in Formvar. Tables 2 and 3 list the six sa.rtples of asbestos and were air-dried on glass slides and photographed at glass that fall in the middle ground of carcinogenicity (Otio v magnifications, From the photographs, These groups show several outstanding differences m lono consecutively counted particles were assigned fibre distribution from those shown in Table I; both to the JO ranges of dimension indicated in Figure 1. extremes in the dimensional range of fibres arc re Vsuming that the particles in a given range were presented. For example, the two crocidolite samples normally distributed around the mean sire of that show similar decreases in carcinogenicity; however, range, the total mass of ail particles could he calcul one is composed almost entirely of long, large- ated; and it is the percentages of (he total mass diameter fibre bundles (Table 2); while the other occupied by particles in a given range or size com (Table 3) has nearly half its mass reduced to fibres partment that are presented in Tables 1-4. that are at the very lowest optical range of diameter In the tables, the first entry for each diameter and and length and the rest reduced to panicles too small half of the dutmrterx in the second entry in each row represent particles that are non-fibrous by optical standards. The plus figures below the designated to be recognised as fibres by optical standards. This latter fraction lx of particular importance, since it consists of panicles that are aggregates of submicro- specimen indicate the extent of pleural fibrosis most scopic fibrils with diameters below 0.2 pm and with commonly observed in the rats in each experiment. lengths of S to lOyim. Since these very short fibrils of The rats are being observed for two years follow submtcroscopic size account for the bulk of this ing application. A necropsy is performed on all material and occur in far greater numbers than in the dead or sick rats, and histologic sections are taken non-pulverised crocidolite, it follows that their role from the site of treatment and from any other ab normal lesion. in carcinogenesis is probably negligible (Stanton A Wrench. 1972). The distributional array of fibres in the three glass samples of these groups indicates the second dimen CONCI.L'SIONS sional parameter of carcinogencily. Here it is appar ent that carcinogenicity decreases as more fibres The data are arranged in four tables according to exceed 2.S um in diameter and as the fibre length our estimates of mesothelioma incidence. In Table I decreases to less than 10 um. Again, the suhnncm- M "ST0095773 K Rf *1 # ir ,|r m Mil'll '! TtW# 2 1</M '*c*/i* - go-jf>5 '30 ??* 0 :r 4t, - ;; c . <0 ?*, ^ 2 ' 70 4<i - ' 0 20 < O'fWHtl* iofiy fv* <0 : 2S S I -os ;s: 2 ; >0 40 1 TO 70 * AAA Q-*ss t* . * <0 ' ... . 7 *> S os ?s< f *'* 4 : V0 40 , 10 ?0 1 AAAg<<<o*'f'! *> 10; 255 . OS 7 S' * TO *3 3 1 - 20 40 . TO -20 i 5 10 ( 25 5 s .05 25, i ; A A * * Irt4t o' p<*u'*f f'fHOAi* Tint' CO^V)H*f P**'rxfjg* o* rn*\ occuo*#d bv &** n Me*' rg# of . jm 70 4T .4,5 *5 *0 *'V *'V VO I 1 : 2: u* 4 t7 ^ 7 1 10 ft 8 1 70 \* 17 14 A T f; 10 ^ p TO .1 wopic fractions .>f these samples contain an ahundant r ti( hbfrx txrluw 11 ^ mn in .) 1,11 nr irr. w till distribu tions of length similar to those ol lihies present in the lowest optical range f inally, Tahle 4 lists those materials that have yielded less than 5*. incidence of mesotheliomas. No tumours have developed in rats exposed to either crocidolite or chrysotile pul'ensed to the extent of reducing all of the optically visible particles to nonfihroux form. Since these excessively milled materials contain all of the elemental components of asbestos and are contaminated by metallic erosion of the mill far more than are the other samples, :t follows 'hat neither of these factors is a likely source of asbestos carcinogenicity and that structural integrity of the fibre is essential. The two glass samples m this group again represent both extremes m structure, namely either fibres of great length with diameters greater than lOumorpanklesthat areshort.thick and essenti ally non-fibrous by optical standards In the nonfibrous Al,0. experiment, a single iqmour has been observed among 30 rats. This tumour is of doubtful significance, but it mnv reflect the low background incidence of less than 5*,, induced by partial frag mentation of the vehicle. Ill Miiimi.tty, die experiments may is- < ompared among single types of material. The results from the seven samples of asbestos indicate that none of the three extremes in fibre distribution yield as high an incidence of mesotheliomas as do the more evenly distributed UICC standard reference samples. Either progressive pulverisation to non-fibrous form nr preservation of the test sample in large bundles of fibres cearly reduces carcinogenicity. In comparing the seven glass samples it is apparent th n samples composed over 90", by weight of fibres with dia meters of 2.5 inti or less are the most carcinogenic; and that as length is reduced in these small fibres carcino genicity is also reduced. Finally, the contrasting results obtained using fibrous and non-fibrous forms of AtjOi re-emphasise the importance of structure in carcinogenicity. These exceptionally pure, inert fibres, composed of materials foreign to asbestos and glass, seem to carry the same carcinogenic hazard for the pleura as do those materials. It would seem therefore that carcinogenicity is in i 1 r '*^g rfi 4? // t Sv partial fraglav rw compared e results from (he that none of the vieUl as high an the more evenly fnence samples, non-fthrous form large bundles of *. In comparing rnt that samples fibres with diacarcinogcnic, and .11 fibres carcinoihe contrasting on-fihrous forms ce of structure in ally pure, inert t to asbestos and mogeiuc hazard rcmopenictty is in v>w frintni.K'M. cosMt>t4Ti>.s n niwr raariNnrusi.sis Te>l* 1 .rod*<t 0O'*n* <70 *0\ 40 ;o ' c-. '* r**'-* /, .'******: 0rmg o CXCup^d b* *>** *n r*. * I '> JM1 y" . 4C e> w* ?o 40 yi ?n 40 %i) 70 1 AAA g-im d*#,,v pj<**fv?<i > 10 I ... | -?V s j 0 7 *> .70 4H . <0 20 5 tC j 76 5 | <04 2 *i . 70 40 j . 10-20 % <0 i * r* * i f, r ; r- .1 4 3 I i inrtif *r* nf mo tomroN oftiwif4 16 22 64 I fthi# 4 N#glt<jhlA tnciffAc* group* (* 5% m#*otbo*0i**) P4*cntgg* of mat* occupy bY m 44Cb rgng* of *f# 0<#mrnr | __-------------. __ _ ym * ^ > h ' - J S 5 20 40 . 10 20 *0 to '25 s OS 25 ! C-o* *<1nf* `tyJly pulvn4<1 : 40 70 40 10 JO * V 10 2S 5 OS 25 Chfyv)il# Mfy CHftv*f**1 4 5 3 T1 * 70-40 to 20 M0 :s s 'OS-25 Comment' Qis wr+tplg fttx# *20-40 >10-20 >s-to > 2.5-4 > 0 S-2 5 AAA gtM *p4rri + 3 '5 3im S 2 -20 40 to JO '5*10 ' I S-S OS J 5 Alurntntym * j nrto ftf)rois% 5 *' s to 14 JS 2 .1 6 9 j 10 i 10-20 20 j* S *1 <1 2 20 1 31 4 md<t !** of plu't< fitKOVt molt <mmor*ty Obwvtd ym -20-40 30 . 40 80 : ; ! 1 | 80 160 * 160-32C ! -1 _ j j -t I ..J1... 2S 1 4 14 2 j e1 ! 28 i 19 j l i 47 24 21 7 1 r>iiygra 7 he pathogenesis of tumors foi'o mg 'hr infraplcurvj! micsfion of asbestos and m\ KirrfihofnfY > f /?r 'ptroftfr t f jff' \ I.< r " Monograph Series. 21. t Kulge. Tenne cc ' ' i !4Mee %p '4 * t S y 1 ^*r * 'A jgner. J C . Hc'rs , (. \ T imhrclr. ^ < I /"<;? Me'* linm.i' <n rats l iio* mg < 'ic ntr.ip cur Jl <n sum '" ashrs'os In Shapiro If \ , cJ /*"-m- prnrffiii^ifX !*<>''nfi! mnaf ( J rr/- SmP. {\jpp : nMo'd I ni\C"- '< <"*'