Marion D. Francis and Rosemary L. Centner The Procter and Gamble Company Miami Valley Laboratorles Cincinnati, Ohio 45247
The Development of Diphosphonates as Significant Health Care Products
Scientific discovery often does not occur as a logical progression o f a single series o f events. Frequently ideas are generated by the sometimes coincidental joining o f several different research trails. . A breakfast meeting to exchange information l e d . .
..
.
Frequently the way in which a scientific discovery is made is as interesting as the scientific discovery itself. The somewhat devious route by which the phosphonates have been shown to have therapeutic, prophylactic, and diagnostic applications in the medical field is documented here as an example of such a story. T h e phosphonic acids are organic compounds containing one or more
2) Condensation reaction
0
-
0
11
+
4H-P-OH
I
11
heat
2CH,-C-OH
0
-!/ OH 'OH
erouos. T h e comoounds of ereatest interest to us are the gemha1 diphosphonates, that is, compounds containing two ohosohonic acid erouos . on the same carbon. The simolest of these is methylene diphosphonic acid H\
I
H/('\Po,H.
CH3-C-0-P-OH
Compounds investigated most extensively are the dichloro substitute of this simple acid and ethane-l-hydroxy-l.1diphosphonic acid, CI2C(PO3Hd2 and CH&(OH)(PO3Hz)z, respectively.' In this paper we will discuss the history of the phosphonates, their physical-che?nical action, and finally their physiological and medical function. Because ethanehydroxydiphosphonic acid was the earliest phosphonic acid investigated by us, we will trace the pathway from its synthesis in 1897 to its current use, particularly as its disodium salt (EHDP), in treatment of disease processes related to calcium metaholism. Historical Development Procter and Gamble first became interested in diphosphonates in the early 1960's, when a research program designed to find better detergent additives showed these compounds to he equivalent to, or better than, most of the additives then in use for complexing the calcium and magnesium in hard water-these alkaline earth metals are the culprits of "bathtub ring" and "tattletale gray." Diphosphonates were first synthesized in 1897 by Von Baeyer and Hofmann ( 1 ) . Suhseauent to this original research. numerous compounds and iy;lthetir pocedu'res have heen reporred (2, >).-Oneof r ht iimolest methods of synthesis devrloped for EHDI' ustls phosphbrous trichloride a i d acetic acid & starting materials and can he shown conceptually as follows
1) Formation ofphosphorous acid 0
I1 3 CH,-C-OH
+. PC!,
3) Hydrolysis reaction
/POSH:
-
0
II H-P-OH
760 1 Journal of Chemical Education
I
0
II + 3CHrC-CI
11
I I O=P-0-C-CH,
O CH, O
t 2H20
II I I
2HO-P-C-P-OH
I l l
I
I
OH OH OH
4) Neutralizationstep
0
CH, 0
II I II
HO-P-C-P-OH
+
2NaOH
O CH:, O
I I I
HO-P-C-P-OH
+
2Hi0
This method lends itself to easy isotopic substitution, e.g., 32PC13, zzPClg, CSH3COOH, CH314COOH, in the starting
'
A simple compound such as ethane-1-hydroxy-1.1-diphosphanic acid with the structure: CH, ,PO.,H, C HO' 'PO,H~ can be named in a variety of ways. The nomenclature above is that must commonly used in the literature and has some advantages not possessed by the radical method, the accepted British and American nomenclature for organic phosphorus compounds. The accepted Chemical Abstracts name for this compound is: (I-hydroxyethylidene) bis(phosphunicacid].
materials. Taeeed c o m ~ o u n d sare valuable tools in pharmacokinetic studies and have besn used in many of our metabolism. ~harmacolopv,and mechanism studies. ~ltho"gh,as just mentioned, the diphosphonates seemed to he useful new detergent additives, the startling discovery of their broad physiological effects, which will he elaborated subsequently, and their biodegradability properties rechanneled the emphasis of research on these compounds into the dental and medical fields. Subsequently, the potential medical benefits of the diphosphonates overshadowed their use as detergency additives. Concurrent with this deterezncv. .oroeram. - . Procter & Gamble (P&G) was carrying out intensive studies on the fundamental nature of tooth decav. We were looking.a t tooth structure, especially the nature of surface reactions to provide protection against acid dissolution by oral bacteria, the essential process in tooth decay. This program had led to the development of fluoride compositions for optimum protection d dental enamrl and the furmul:~tionof a itallnous fluoride toothp:~stein 19% ~Cres~"'). Sta~lnousflu,.r:dr trmlmmt of tooth-enamel produces an extremely thin layer of densely packed particles of calcium fluoride and stannous phosphate and forms an effective protective surface film on dental enamel. Moreover, unlike acid treatment with sodium fluoride. there is minimal destruction of the enamel during treatment (4). These studies provided much of the fund; mental chemical backeround for subsequent research on the reactions of the phosphonates with hydroxyapatite (HA). Another area of research relevant to this history, also being carried out concurrently, was a search for calcium chelating compounds which could preferentially remove calculus (tartar) from teeth without damaging (dissolving) the underlying enamel. This posed an unusually difficult problem in that calculus is essentially composed of HA as is enamel. Among
the numerous calcium chelators examined, such as ethylenediamine tetraacetate, EHDP did not produce surface damage due to dissolution of the enamel a t either acid or basic oH's (Fie. .. 1). . EHDP. however. was not able to dissolve alreadv depusiud c ~ l c u l uin~an in w t n , model syrtem. We found that the hlockinc of d ~ i i d u r i o ng ~ HA i h\' KH1)l'and certninother phosphonaies tested occurs because of the formation of an extremely thin insoluble surface film on both the calculus and enamel apparently somewhat like that provided by stannous fluoride. This uncharacteristic behavior of a "strong" chelator led us to carry out physical-chemical studies on calcium phosphate formation in the presence of EHDP. Usine soecific titrimetric methods we found that. in the of EHDP, formation (nucleation) of HA was occurrine. hut crvstal erowth of calcium . phosphate (HA) was . being Eiocked c5). These observations convinced us that removal of calculus already deposited on the enamel surface without damaging the tooth would he extremely difficult or impossible; however, we could perhaps prevent formation of cal&lus by blocking crystal deposition while still maintaining enamel integrity. In a specially developed animal model of calculus (6)EHDP inhihited the formation of calculus and protected the enamel surface. With additional experiments we determined that the effects observed were topically induced, not systemically induced (7). . . Subaeqtwntlg, ;I hyp,rthesi; was proposed to explain these ~fft.,.tohos~hntc iuld thp pchyph&phates, he stro&ly recommended that we get together with Dr. Fleisch, and wrote Dr. Fleisch about our conversation. Dr. Fleisch then wrote to tell us he was coming to the US. and we subsequently invited him for a visit to discuss our mutual interests in calcium phosphate chemistry and metabolism. This meeting with Dr. Fleisch on June 28,1966 and suhsequent work at his institute and a t our Miami Valley Lahoratories has produced a great broadening of our understanding of the effects of phosphonates in calcium and phosphate metabolism. Subseqia*ut world research irrrerrst has resulted in more rhm :310puhlirations. Figure ;I summarizes the merger nithe various research pathways leading to the discovery of the medical and dental applications of diphosphonates. The most important discovery that we made was that both the geminal diphosphonates and the condensed phosphates behave similarly in their effect on calcium phosphate in uitro, but in uiuo the P-C-P bond of the phosphonates is hydrolytically stable, while the P-O-P bond of the condensed phosphates is unstable. This inherent instability of the condensed phosphates was suhsequently shown to he enhanced by the action of enzymes always present in the in uiuo system (13).
.
~
~
Condensed phosphates thus do have limited effects when injected directly into the blood stream, hut when introduced into the body by other routes (i.e., ingested orally or injected hv other narenteral routes) are hvdrolvzed hv natural suhstances inthe body (i.e., phosphat&es and acids) before they can reach their site of action. Diphosphonates are effective regardless of their route of administration, although by the oral route only about 1-6% of the administered dose is ahsorbed (14). What are some of the physiological consequences of administering compounds which can have a long-term effect on calcium metabolism? What is their probable mechanism of action? The most exciting question to be answered, of course, is what are the possible therapeutic or prophylactic benefits to man of com~oundswith such an astonishine resistance to rnzyn~atirattack and w ~ t hthe potential (ria reaer\uir effect from their ndsorvtion on the ikt~lvtm?Hefurv we diwuss t h ~ s e answers in detail, we need to look more closely at the fundamental physical-chemical hehavior of diphosphonates in order to understand the way in which they may affect biomedical processes. Physical Chemistry of Diphosphonates Basic to insight into the hehavior of the phosphonates is an understanding of their polyprotic nature and ability to chelate with cations. The acid-hase titration curves of EHDP alone and in the presence of Ca(II), Mg(II), and Fe(II1) (Fig. 4) demonstrate that the first two hydrogens are titrated simultaneously with an endpoint at a ~ ~ r o x i m a t.eDH .l v 5.5: the third hy(lruyei is neutraliz(&lat abou;~..'.'l'hr I w ~ r t hhydrogen in the ohsvncv of metal cariun cunnigt he neutralizpd. Metal cation complexation by EHDP is responsible for the displacement of the fourth hydrogen and has formed the basis for analytical methods of determining EHDP quantitatively ( 1 5 , l f i ) .In the presence of metal cation, the fourth hydrogen is either partially neutralized [Ca(II)]or completely neutralized [Fe(III)] if concentrations of phosphonate and cation are kept below levels that produce precipitation (Fig. 4). The ionization constants of EHDP (pK1 = 1.4, pK2 = 2.8, p K 3 = 7.0, ~. uKd = 11.1) and other .ohos~honates have been deter. mint4 IIVa numher o r investigators (17-201. Wc have already discussed the eiiect ot KHDP in hluckinp the growth of newly forming HA crystals in chemical solutions F:xpcrimcnts on the distril~ution( r i KHI)I'-l-14(: hetween precipitate and fillrnte rdcslcium and phtrsphate s.vsremi (21J ha\,e indicawd that the KHI)P is strongly c h e m i s d ~ ~on . d the crsstallitei of HA. This s~ecificadit~rt)tiimrewlrs in hishlv - " dichotomous actions of EHDP and other phosphonates on
Mlai O104N NaOH
Figure 3. Network of research fields and observations leading to the application of phosphonates lo human clinical use.
762 / Journal of Chemical Education
.
Figure 4. Variation in p H a s a function of the titration of 100 ml of0.005 Methane-l-hydroxy-1,ldiphosphonic acid with 0.104 Msodium hydroxide in the presence and absence of 0.005 Mcabium. magnesium. or iron. EHDP acid alone (2 separate tinatians), 0 EHDPacid piusCa2+. A EHDP acid piusMg2+. D EHDP acKl plus Fez+. 0 EHDP acid plus Fe3+.
What are some o f the physiological consequences o f administering comoounds which can have a long-term effect on calcium metabolism?
HA. If EHDP is present when ralrium phosphate nurleaws, only particles of about 15-100 radius are iormed. L'nder thwe conditions, an incrvase in apparent solubility of HA results (22). In contrast to the abure incrra,w in apparent soluhilitv. if~ EHDP r~ uf HA ~ ~ -is oresent ~ ~ af ~~ e marrorrvstalliws have already formed (particles with the size550 X 50 X 200 A). a decrease in solubilitv rate of HA results (23). This decrease in rate of soluhilit~isdue to the adsorption of EHDP on the HA crvstallites, decreasing the surface area available for dissolution and so decreasing the rate of dissolution. Both of these effects, however, are caused by the same mechanism, chemisorption, occurring a t a different stage of HA growth. These apparently opposite effects on solubility of apatite have important implications on pathologies involving both unwanted deposition of calcium salts in soft tissue and unwanted loss of hone mineral. Physiological Behavior In Living Systems (24,25)2 Soft-Tissue Calcification The first verification of the systemic activity of the phosphonates in living systems was obtained when our diphosohonates were tested in Dr. Fleisch's animal model of aortic calcification produced by administration of excess Vitamin Da in rats. (This was the model Dr. Irving had described to us inwhich he showed the intravenous activity of the polyphosphates.) As we had predicted (based on the stability of the P-C-P bonds and their cbemisorptive reaction with calcium phosphate), the phosphonates blocked aortic calcificationand dewneration both narenterallv and orally. - ~ - - - - ~when ~ ~ administered ~ ~ ~ Subsequent to these experiments, specific studies on the enzvmatic cleavine abilitv of the acid, neutral, and alkaline ihosphatases confirmed that the P-C-P bonds were physioloeicallv stable. In other model svstems of ectopic calcification which >ere tested, such as ~ e l ~ e' ' C s a l ~ i ~ h ~ l a x iusing s" dihydrotachysterol, the phosphonates were likewise found to be effective in blocking undesirable calcification of soft tissues. In all the svstems tested, the major structural requirement for activity isthe P-C-P arrangement, i.e., geminal diphosphonates. We also found that polyphosphonates of the type ~
where n is three or more are also effective. Compounds which tend to he ineffective, depending upon the presence and position of other functional groups, are the monophosphonates and vicinal diohosnhonates of the above tvne . . .. where n eauals two. A gogrl curn4iltion exists between the ability ot indn.idual tu inhibit rrvstal in ~ i t r and o to inh~bit .ohos~hmates . . growth soft tissue calcification in uiuo; in fact, the in uitro crystal growth test is a good predictor of in uiuo effectiveness. Bone formation occurs by the deposition of mineral salts, primarily hydroxyapatite, in an organic matrix, e.g., cartilage. At high doses, EHDP inhibits cartilage mineralization in growing animals hut does not prevent the development of the organic matrix itself. When treatment with EHDP is stopped, the organic matrix then mineralizes. Likewise, EHDP inhibits ZThe~etwo recent reviews contain citations to the work described in the sections "Physiological Behavior in Living Systems" and "Human Clinical Trials".
the mineralization of decalcified hone that has been implanted into the soft tissue of rats. Interestingly, disodium dichloromethane diphosphonate (CIzMDP), which is as effective as EHDP in preventing soft-tissue calcification, does not appreciably inhibit cartilage and bone mineralization. Bone Resorption Because diphosphonates in solution not only can inhibit crystal growth hut can also considerably delay dissolution of already formed apatite crystals, our studies included an examination of the effect of phosphonates on in uitro and in uiuo hone dissolution. Initially tissue culture studies were carried out in which mouse frontal skull bones (calvaria) were explanted and bathed in various media that stimulated bone resorption. Addition of diphosphonates directly to these media or administration to the animal prior to explanatation retarded the hone dissolution processes in the cultures. In uiuo studies were also carried out in models desiened to indute excessive bone loss. In general these models involved immobilization of a hind limb by nerve severance (simulating paralysis and a type of osteoporosis in humans). In these models the immobilized limb loses bone mass compared to the contralateral normal limb. The phosphonates, administered either ~arenterallvor orallv, reduced hone loss - significantlv in the immobilized limb. All the above studies suggested a direct effect on calcium and bone metabolism. Normal bone is a living tissue undergoing constant resorption and redeposition of calcium, with the net effect of maintenance of a constant mineral balance. In growing bone, mineral deposition exceeds mineral resorption. In certain oathological conditions. bone resorntion exceeds hone deposition iesulting in osteoporosis. ?he dual orocess is commonlv called "bone turnover." Kinetic studies carried out in rats iabeled with 45Ca showed that the hone turnover rate was affected when diphosphonates were administered. Increasing doses of either EHDP or ClzMDP progressively block hone loss induced by a low-calcium diet. The mineral content of bone and the net calcium balance of the animal increases only slightly, however, because bone formation rate is also decreased. Both of these diphosphonates have a similar effect on bone turnover at lower doses. At very hieh doses. EHDP treatment results in the accumulation of &ssivc osteoid (unrnineralized hune matrix) and l,w~!re~l miueral content ui bun? and cartil,wr matrix whereas CI ,MI)[' retards the remodelling of bone (turnover process) without the resultant formation of excessive osteoid and lower mineral content. Calcium kinetic studies such as those above provide an indirect measure of hone activity. Bone morphoiogy, however, provides a technique for visual confirmation of the changes In bone. The diffeiences in appearance of bone after administration of high systemic doses of EHDP and ClzMDP are shown in Figures 5 and 6. CIzMDP retards the remodeling (reshaping) of the growing end of the bone or metaphysis and becomes radioloeicallv .. , more dense than the control. thus demmstrating inhilution c g f reimption and lark uf innilit ion m~ncrillizatio~~ FHI)I' hlocks mineral d w u ~ i t i o nand results in a widened epiphyseal growth plate'(a thin layer of cartilage at the end of the metaphysis of a growing hone) compared to control and decreased radiological density due to decreased mineral content. The doses of phosphonate that elicit these responses (Figs 5C;D, and 6C, D) are almost 40 and 100 times the systemic dose absorbed by man from therapeutic doses of EHDP. Volume 55, Number 12, December 1978 1 763
Figure 5. Microradiographs of
tibiae from
control rat ( A ) and rats treated with
diSOdium ethane-l-hydroxy-1.1-diphosphonate at 1 (8).10 (C), and 30 ID) mg
Plkglday subcutaneously for 14 days. (Reproducedfrom search. 11, 196 l1973l.l
Calcified Tissue Re.
Figure 6. Mlcrorad~ographsof tibtae from control rats ( A ) and rats treated with disodium dichloromethanediohos~honate at 1 15). 10 IC). and 30 ( 0 )mQ .Plkgtday , . subcutaneously for 14 days. (Reproducedfrom Calcified Tissue Research. 11, 196 (1973))
. .
Diohos~honatesand Vitamin D Metabolism Vitamin D3 (cholecalciferol), which is ingested in food or vitamin suonlement and is formed bv the action of sunlight .. un the skin, 19 ~ransfurmedin the li\:er to 26-hydruxychhlecalt:ifenL This metabulitr is then transported to the k~dnt:.v, where it is again hydroxylated to form i,25-dihydroxycholecalciferol (1,25-(OH)7D:d. This latter compound is thought tu be the actiw m r t a l r ~ l ~u,hich tc is respons~l)lefur controlliog the rate oicnlcium ahsorptiun from tht: intestinal tract tu meet rhr calcium requiremenrs of the organism (Fig. 71 ( 2 7 ) . The effect ut diphusphunates on rht: vitnnti~~ D-calcium interr~lar~tmshiv has heen studied extensivelv. I'H [)I' has an interesting biphasic effect on calcium absorption and vitamin D metabolism in rats: At low doses. EHDP slinhtlv increases the abwrption of c;ilcium from the inreitinal tr& and increases the content u i 1,25(OH11Dtin the intestinal tissue of the rat. At the high systemic doses-at which EHDP has been shown to inhibit bone mineralization and produce excessive quantities of unmineralized osteoid, EHDP reduces the intestinal absorption of calcium and intestinal content of 1,25(OH)*Daand completely inhibits the production in the kidnev of the 1.25-(OH)?Da in both the rat and the chick. o r 2 administratkn of EHDP a t therapeutic doses in man has been shown to increase gastrointestinal calcium ahsorption and to shift the calcium balance of patients in the positive direction (27). This presumably is related to the 1,25(OH)zD3 production and to bbne resorption and mineralization shifts with administration of EHDP. In contrast.. CloMDP. which does not lead to excessive ns" teoid formation when given a t very high systemic doses, does not i m ~ a i calcium r ahsorntion in the intestine (28)and does nrtlwry nut inhilit 25-hy~lroxvrh~~lecalciferol-I-hydroxylasr in the kidney (29) in chicks maintainrd on a diet adequate in vitanti~tD rrmtent. 'l'heie diiferences in heha\,ior hetween EHI)l'and (.'l,,hlDP a i l 1 ultimately dfstermine thrir wet i i ~ c uses in variois disease processes involving calcium and phosphate metabolism. 764 / Journal of Chemical Education
melaoo sm of Lnolc~alc8feraor V lam n DJ ID,) The n y ~ r o x ~ l a t m(-OH1 n of Dl lo 25-0hD3 occurs n the ver The nyarorylatmn of 25 O m 3on m e k dney can y e d 1.25 Ohl,D, oy reactm ( tor 24 25-10nrD~ by reaction (11). Reaction I appears to predominate in calcium deficiency and under conditions of a greater physiological need for calcium and reaction (11) predominates when reaction (I) is depressed as under high levels of dietary calcium or strontium. (Reproducedfrom Vitamins and Hormones. 31, 45-47 F~gure7 Tne pr mary
(19731.1
Toxicology
Concurrent with the animal efficacy and mechanism experiments just described, we have carried out extensive toxicological studies with EHDP and ClzMDP. Acute, snhchronic, and chronic administration of these phosphonates in several animal species has shown them to have very low toxicity (30).
The first patient to whom EHDP was given was a two-year old girl
...in whom calcification o f the muscles controlling respira tory
movement o f the chest cavity was threatening her life.
Both comnounds are auite stable in vitro and neither compound has'been shownio he metabolized in the living system. At excessively high doses which produce death, the mechanism of lethality has been shown to he complexation of ionic calcium; this effect can he completely alleviated by the parenteral administration of an ionizable calcium salt such as calcium eluconate (31). Additional animal studies have shown that these phosphouates are not teratogenic, mutagenic, or carcinogenic. From the manv, animal studies carried out we were assured that intnid~lctionof these phorphunates into human clinical trials carried a verv low rlt!ment of risk. When administered to normal human subjects, EHDP, as predicted from the animal studies, was well tolerated a t therapeutic levels. ~
~
~~~
~~
~
Human Clinical Trials The impetus behind all the chemical and animal studies carried out on the phosphonates was the underlying concept that fundamental disturbances in calcium and bone metaholism are resnonsihle for manv human natholoeical conditions. Examples 01 th( a,nmditinns are the pntholoyical depositiun of calcium salts in soft tisue which occurs in pnral\~icp:itlcnts major surgical junu replacement and i l l patient.; ~~ndt:rytrir~y and I'arct's disease (I!' hone a hich is characterized l)v rapid resorp$on of hone and deposition of abnormal hone. The first patient to whom EHDP was given was a two-year old girl with severe myositis ossificans progressiva (MOP) in whom calcification of the muscles controlling respiratory movement of the chest cavitv was threateniua her life. Dr. Bassett of Columbia presbyterian Hospital in New York had learned of our researchon EHDP throueh Dr. H. Fleisch, who suggested that Dr. Bassett call us. At t h k time we were not yet ready for full human clinical trials. However, the extraordinary nature of this case, together with the ethical and compassionate considerations associated with it, convinced us to cooperate with Dr. Bassett. He obtained the necessary approvals from his institutional review committee and the Food and Drug Administration and began treatment. The girl showed remarkable improvement, and is still alive a t the time of this writing. Due to the rarity and cyclical nature of MOP, conclusive clinical proof of the efficacy of EHDP in this disease, however, has not since been obtained, even though a number of additional patients with the disease have been treated. Based on chemical and animal studies, and supported by the clinical improvement in Dr. Bassett's patient, we were convinced of the potential usefulness of EHDP in diseases of calcium metabolism. Our problem now was to focus on the right disease in man in which to test this concept. Discussions with Dr. R. G. G. Russell, of the University of Berue and Dr. R. Smith, of the Nuffield Orthopaedic Centre in Oxford, England, led to probe studies in Paget's disease of bone. In these studies, we observed the striking biochemical changes and therapeutic effects elicited by EHDP in Paget's disease. In addition, we also conducted studies in the U S . on the clinical nharmacoloav of EHDP to establish its safety and metaholk effects innormal subjects. Subsequent clinical trials in more than 700 Pagetic patients conducted over a period of six years in hoth U S . and foreign clinics have established that EHDP when given orally is both safe and of sienificant theraoeutic benefit (32).In other trials in patlents undergoing hip replacement surgery and in patients sufferioe from . oaralvtic . spinal . cord iniurv. " .. EHDP has been shown to he effective in reducing pathological calcifi~
~
-
cation of soft tissue (33). In addition, human clinicals havt verified the ability of EHDP to prevent the formation of dental calculus whether applied topically in solution or incoroorated into a dentifrice. During the development of the basic understanding of the phosphmnte chemistry and physiology, one of 11s(11DFJ was invited by l)r. I). Van Dvkt of Donner Lnhnratorie* to speak on the r~hosr~hunaws at the Berkeley camous. Subsequentlv, the ~ o n n e r i a h o r a t o r ysuggested another interesting a p p k cation of the phosphonate, EHDP, namely in the diagnostic use of an aqueous complex of EHDP and technetium-99m in the presence of tin(I1). We sent EHDP to Dr. Van Dyke and after independent in uitro and animal studies at hoth Donner, and P. & G., the aqueous complex was formulated and prepared at Miami Valley Laboratories and immediately flown to Berkeley where the first clinical proof of bone scanning efficacv in humans occurred. This comnlex with its 140 KeV-y-emission is very efficient in concentrating in areas of increased skeletal turnover such as occur in Paget's disease and bone tumors. It is also used to detect thepresence of mvocardial infarction in patients suspected to he victims of heart attacks. The bone scanning agents concentrate in the infarcted tissue of the heart because of the calcification which occurs in the damaged tissue (34). EHDP itself can he labeled with 32Por 33P;the 8--emission from either of these comoounds. which would concentrate a t sites affeded by malignak bonetumors, could possibly affect the tumor without causinedamaee to normal tissue. Thus the " pervasive pain associated with such tumors might he alleviated. or nerhans the nroeress of the tumor itself could he slowed or halted. clinical Gials are in progress to assess the benefits of such treatment; animal studies have already defined the safety hazards and potential efficacy of this approach (35, 36). Future Research Areas Many additional questions have yet to he answered on the further annlications of . nhosphonates in medicine, as well as .. . on their fimd;unent;il mechanism ofactiun in I i ~ % .A numhtr of d~ieasesinvolving calcium merahdicaherrationsoifer additional opportunities for the study of phosphonate prophylaxis and therapy. Osteoarthritis is a disease which is characterized by the accretion of bony, spur-like projections a t joint interfaces, accomnanied hv, oeriodic attacks of wain and inflammation. . Tht: infl:immatury aspect oiostroarthrnis hn.i been suggested 10 11v du? to d~atitic!{':I or calcium r~vro~husohale ." . . dihvdrate (38) crystalsLforming in the joint fluids. "Lipping";n 0steoarthritis is a normal accretion of honv tissue in an abnormal alntnmt and site cauiing hindered joint mution and frequently Ithrumntuid arthritis is another t'llrm o f w n t d ~ s ~ a i n1.791. Lase thought to be systemic in nature rather than a local process, as is osteoarthritis. Rheumatoid arthritis is characterized by inflammation and destruction of cartilage and hone of joints and by a progressive deposition of calcium in and around the joints which ultimately results in loss of joint function. Atherosclerosis is a complex process involving the slow build-up of a plaque (a deposit on the inner surface of hlood vessels) in hoth major and minor arteries, resulting in inhibition or hlockaee of hlood flow. Consistenth. oresent . withm this plaque (containiny lipids and fihmus pruteini like elaitin and cullnrenl IS a calcium ohosohate deposit which may provide thematrix for the ac&uiation ofthe organic deposit. All of these disease processes involve, in some way, the deVolume 55, Number 12, December 1978 / 765
