Bioinorganic Activity of Technetium Radiopharmaceuticals Thomas C. Pinkerton. Carla P. Desilets, Daniel J. Hoch, Martin V. Mikelsons, and George M. Wilson Purdue University, West Lafayette, IN 47907 Technetium radiopharmaceuticals are diagnostic imaging aeents used in the field of nuclear medicine to visualize tisues, anntwnical structures, and metabolic disorders. The isotuneof technetium utilized in nuclear medicine in technetium:99rn. The metastableggmTcis short-lived (tllz = 6.02 h) with a predominant single-photon y emission having an energy of 140.6 keV. After intravenous administration, 99mTc radiopharmaceuticals localize in specific target tissues, which can then be imaged using sodium iodide crystal cameras. A wide variety of tissues can he visualized with -T "'c radiopharmaceuticals, including the kidneys, bones, lungs, heart, liver, brain, and thyroid. Although other radionuelides are used in nuclear medicine, of the millions of diagnostic imaging procedures conducted each year, over 80% involve the use of 99mTc.Even with the advent of more sophisticated imaging procedures such as Positron Emission Tomography (PET) and Nuclear Magnetic Resonance (NMR) imaging, technetium radiopharmaceuticals remain the workhorse of nuclear medicine. Technetium radiopharmaceuticals will maintain this position for some time hecause they are more readily available and cost less than other imaging procedures. Recause the half-lives of technetium's most stable isotopes are short on ageological time scale, technetium is not a naturally abundant element. Indicated as element 43 on the periodic chart, directly under manganese, some of its properties were nredicted bv Mendeleev in 1869 (1).He called this yet undiscovered element ekamanganese and gave it the svmlxd Em. Noddack and Tacke (21 claimed LO have isolawd i e m e n t 43 in 1925, hut Perrier and Segre (3)are universally credited with its discovers in 1937. They produced weighable quantities from trankmutation reactions of deuterons and protons with molybdenum. After World War 11, Perrier and-Segre gave element 43 the name technetium (pronounced tek-ne-she-m), from the Greek word technetos (meaning artificial), to commemorate i t as the first artificial element. Amone the lone-lived isotooes. the most readilv available = 292 keV). since the first is 99Tc (illz = 2.G X 105 y, gram quantities of 99Tc were made available by Cobble (4) in 1952, the chemistry of technetium has been studied by comparatively few investigators. The chemistry of technetium is similar to its neighboring transition elements, manganese and rhenium. Technetium gives rise to multiple oxidation states and forms coordination complexes with a variety of inorganic and organic ligands. Many of the thermodynamicall; stable technetium~compoundshave oxygen hound to the technetium (e.g., TcOa-, TcOy2Hz0, TcOL, etc.). Because technetium can form a wide ranee of stahleggmTc complexes, its use in nuclear medicine has grown enormousIv in the nast 10 vears. Todav 99mTcis conveniently produced in hbspitals br local nuclear pharmacies by the Lfdecay of 'j9Mo (Fie. 1). Molyhdate 99MoOa2- is adsorbed onto alumina columns encased in lead-shielded. nortable containers called aenerators (Fig. 1). On the daiAofuse, the 99'"Tc, in the Form of pertechnetate (TcOa-), is eluted from the generators with saline solution. In most cases, the solutions of pertechnetate
o,,
~~~~
~
are simply injected into sealed vials containing a lyophilized mixture of a reducing agent (usually stannous chloride^. the comnlexine lieand. and nerhans an antioxidant stabilizer. ~he'redox"re&ent reduces the technetium from a Tc(VI1) oxidation state to some lower-oxidation state (i.e., T O ) , Tc(IV), Tc(III), or Tc(I), depending on the reducing agent), followed by technetium complexation with the ligand. The technetium complexes exhibit a variety of stoichiometries d e ~ e n d i n aon the oxidation states. The Tc(V) and Tc(IV) complexes are believed to have technetium-0x0 cores (i.e., TcOL,, TcOzL,, TcO(OH)L,, Tcz02Lnetc.); whereas, the Tc(II1) and Tc(1) complexes are known to have the ligand coordinated to the technetium alone (i.e., TcL,, TczL,, etc.). Sincr technetium can form multiple complex species, it is not uncommon for a reaction mixture withonly one ligand to form many different complexes. The polynuclear formation of technetium species is usually minimized by keeping the technetium concentration as low as nossible. In carrier-free preparation, where 99Tc is termed thk carrier, only t h e S s m ~ c is initiallv nresent. In such nreparations, the 99mTcconcenrange from 10-?to i0-SM. Polynuclear complex tration formation he.. dimers, trimers, etc.) is generally unfavorable at such lowmetalconcentrations. ~ o p r e v e nthe t formation of technetium complexes with more than one oxidation state, efforts are made to control the strengthof the reducing agent and the reaction conditions. More details on the reaction chemistry of technetium can be found in several excellent reviews (5-8). At some appropriate time after the -"T 'c radiopharmaceutical has been prepared, an aliquot of the reaction mixture is administered intravenouslv to the patient undergoing - diagnosis. I t is customary to wait a given length of time after administration before the imaging is actually conducted. If a .hone scan were being performed, an individual would wait about 3 h, in order to allow the 99mTcactivity to clear from the blood and soft tissue and accumulate in the target tissue. The image produced from the detection of gamma radiation with sodium iodide cameras has a somewhat blurred appearance. Because of this low resolution, i t is important to
?Y
saline
"RO;
99m~c GENERATOR 9
u
9
~
o 9~ 9 m~ ~ c 0 i 5 66h
lalumina column)
Figure 1. Technetium-99m generator.
Volume 62 Number 11 November 1985
comolexes readilv diffuse into extracellular soaces throueh achieve the highest possible contrast between the target and the slit pores, driven by the concentration gradient across the background tissues. the caoillaw cell wall.'l'his means that terhnetium comoleaTechnetium radiopharmaceuticals can be divided into es a r e rapidly distributed throughout major parts or the two distinct classes (9). The first class, referred to as the technetium-tamed radio~harmaceuticals.consists of techbody. netium-labeledmolecules whose biodistrihution is deterIn some cases, technetium complexes are reversibly bound to plasma proteins. Since protein-bound technetium commined entirelv bv the soecies to which the technetium is plexes cannot traverse the slit pores, they remain in the attarhed. his 'eans that the native molecules, if not labeled with the trchnetium, would exhibit an invivodistribuhlood reservoir. As the free concentration of the technetium complexes in the blood decreases, either by accumulation in tion identical to the technetium-labeled substances. This the target tissue or elimination from the body, the techneclass generally includes high-molecular-weight species such as proteins, large particulates, colloids, and cells. The techtium complexes are released from the proteins, as the equilibrium shifts. Technetium complexes, like other species netium-tagged class encompasses 9~mTc-sulfur colloids, used to imaee the reticuloendothelial svstem of the liver. soleen. that bind t o protein. clear the blood much more slowlv than and h&e marrow; rna~roaggre~aied all)umin, ""D~TC-%~AA,compounds not bound to proteins. Onre asuhstance has reached theextracellular fluid of the used in lung oerfusion studws; radiolabeled red blood cells. hlood pool imaging; and 99mTc9 9 m ~ c ;sed - ~ ~to~conduct ; target tissue, i t must traverse the outer membranes of the taraet tissue cells in order to accumulate. Cell membranes tamed antibodies, which have been developed in attempts to i&e various soft tissue tumors. These and other types of arecomposed of lipid bilayers impregnated with various technetium-tagged radiopharmaceuticals are reviewed in proteins (Fig. 3). Cell walls contain small pores (-8 A in diameter) through which small molecules such as water (3.0 detail elsewhere (5, 9) and will not he discussed here, since A diameter) and hydrated chloride ions (3.8 A diameter) their biodistrihutions are solely dependent upon the native macromolecules. readily pass (10). Hydrophobic molecules can pass directly ~ - - ~ ~ ~ The second group of technetium radiopharmaceuticals is through the lipid bilayer by passive diffusion. Medium-sized hydrophilic species can cross the cell membrane either the technetium-essential class, which consists of small technetium-ligand coordination complexes. The biodistrihutions of the technetium radiopharmaceuticals in this class are controlled by the physical properties of the technetium comolexes themselves. The in vivo distribution of these radio~harmaceuticalscan vary with the structure of the technetium complexes as aoverned by the technetium oxidation state, functional groups on the &and, and technetium core configuration (i.e., monomeric versus polynuclear). The technetium-essential class includes technetium complexes used to image tissues such as the kidney, liver, heart, and bone. The relationships between the physical properties of these complexes and their hiodistribution mechanisms are ooorlv . - understood. This is due in oart to an inadeauate knowledge ot the chemical structures of technetium romolexes and to the ore\.ious una\.ailabilitv of chem~callvpure technetium compiexes for biodistribution studies. ~ e i e k t l y , however, technetium complex reaction mixtures have been separated by high-performance liquid chromatography (HPLC). Isolation of pure technetium complexes after HPLC has enabled investigators t o begin correlating the physical properties of single technetium complexes to tissue uotakes. These structure-activitv relationshios shed lieht on t i e physiological uptake meccanisms of the technetium comnlexes and nrovide a better understandine of their bioFqwe 2 Cut-away u eu 01 a blood capinw: (A) oioodstream. @I Mwa cell. distrit~ution.This fundamental information aids in the de(C) endothe1a1 cell of capillary wall. (D)slltpars between cells. (E)extracellv lar fluid mmpartment. (F) target tissue cells. (G)gap junctions. sign of neu,, more effective 99m'l'cimaging agents. ~~~~~~~
-
Bloavailabllily of Technetium Complexes In order to understand why certain body tissues are selective to technetium complexes in the technetium-essential class, one must first understand how the agents reach the cells of a target tissue. Because technetium radiopharmaceuticals are intravenouslv administered. thev are immediatelv distributed in the blood pool. Technetium complexes, as a rule. are water soluble. so thev diffuse and flow freelv in the hlood plasma matrix, thus reaching the target tissue in the same manner as body nutrients or therapeuticdrugs (i.e., via the capillaries). The capillary walls are made up of a monolayer of endothelial cells separating the flowing blood plasma and the extracellular fluid which bathes the cells of the target tissue (Fig. 2). Between the endothelial cells in the capillary walls are slitlike spaces which act as pores and which have openings of from 80 to 90 A. These slit pores allow for the passage of small, water-soluble nutrients and drug molecules but exclude macromolecules such as plasma proteins (10). With the exception of the brain, technetium 986
Journal of Chemical Education
0
rn-
@~~
Figure 3. Lipid bilayer ol cell membrane.
through active transport enzyme systems or by means of carrier proteins. Transport by carrier proteins can only occur from a high concentration to a low concentration. On the other hand, active transport processes can move species against a concentration gradient. Substances usually pass through cell membranes by one of the four processes (1) diffusion through small pores, (2) passive diffusion through the lipid bilayer, (3) mediated transport by carrier proteins, or (4) active transport involving enzyme systems. Since technetium complexes are considered to be larger than 8 .& in diameter, transport occurs by one of the latter three means. Once the technetium complexes have reached the intracellular space of the target tissue, they may accumulate in some organelle or other component within the cells. Since tissues are constructed of cell layers, one might wonder how species gain access to cells deep in a tissue. One way is by diffusion of the species in the extracellular fluid. Another way is for the substance to pass from one cell to another through the so-called gap junctions (Fig. 4). Gap junctions are pore-like passageways with diameters of 15-20 .&,which join adjacent cells (11). These channels allow inorganic ions, metabolites, sugars, and amino acids to pass from the interior of one cell to the interior of an adjacent cell. Proteins, polysaccharides, and nucleic acids are too large to pass through these channels. The gap junctions are important in providing nourishment to cells that are distant from blood capillaries, as in the bone. The permeability of the gap junctions is controlled by the intracellular concentration of Ca2+. When the calcium levels are below lOWM, the gap junctions are fully open; however, when the Ca2+ concentration exceeds 5 X 10-SM, the gap junctions close (11). The biodistribution and accumulation of any substance in body tissue is a function of its physical properties. In the case of technetium complexes, as with many other small compounds, biodistribution is determined by the species' (1)
+I S - z o 8 + Figure 4. Cell gap junctions between cells.
lioid solubilitv. - . (2) . . hvdrated volume. (3) ionic charee. ( 4 ) piotein binding, and (5) target site affinity. The following is a brief discussion of the current theories surroundina the biodistribution of some technetium complexes in the technetium-essential class, with emohasis on structure-activity relationships. The discussion is intended to be fundamental and not exhaustive. For more details, the rrader is refrrred to two excellent comprehensive treatises on the structureactivity relationships of radiopharmaceuticals (12.13). Thyroid Imaging The localization of pertechnetate (9SmTcO-) ~n ' thyroid is an easily understood process. The thyroid gland requires iodide ion (I-) for production of iodinated hormone, thyroxine. The iodide ion is transoorted across the membranes of the epithelial cells in the 'thyroid by means of an active transoort orocess. The solvated radius of iodide is comaarable td that of pertechnetate. Because each of the ions has a single negative charge, they have similar charge densities. The active transport proteins in the thyroid, therefore, cannot distinguish between the two ions. Consequently, 99mTcOa-is taken up readily by thyroid cells. Even though iodine has several y-emitting isotopes (i.e., '311, 1261, and lZ3I),the [99mTc]pertechnetate is preferred. This is because 1311has an 8-d half-life and emits multiple nonuseful y's and Fs; 1251has a 25-keV y, whose energy is too low for imaging, and lZ3I,with good imaging characteristics (13 h half-life, 159 keV y), is expensive. Brain imaging [9gmTc]pertechnetate is used in brain imaging, as well; however, it is limited to the detection of ruptures, tumors, and vascular lesions that impair the function of the bloodbrain barrier, which is normally impermeable to the highly water-soluble 9 9 m T ~ 0 4ion. The capillaries in the brain are different from those throughout the rest of the body and make up the blood-brain barrier (Fie. 5). The cells of the caoillarv walls in the brain are tightlyjoined together, havingno "hit pores" between the cells. A basement membrane and a fatty, footlike sheath (called the glial foot) of adjacent nerve cells further separate any diffusing species from extracellular brain fluid. All substances must traverse the capillary cell membranes as well as the fatty tissue lining of the elial foot to cross the bloodbrain barrier. Efforts are being made to develop technetium radiopharmaceuticals that will diffuse naturallv across the bloodbrain barrier in order to image various regions of the brain through which blood perfuses during mental activity. The
Flgure 5 Blaod-oram barrier (A) brsm Issue. (BI oloodsbeam. (C) blood celn. (0) cap8llery wall endolhellao cells it!gntly lomeal. (El oasemenl memorane. IF1 glial feet (faw sheath) extending from asbocyte celb.
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Number 11
November 1985
967
imaging technique that would heemployed insuchstudies is call(.d single-photon emission computerired tomography ISPECTI. The technioue involves seouential movement of the sodium iodide crystal camera around the target organ and computer reconstruction of radiation detected a t various angles. The method generates a three-dimensional representation of the target tissue. The rate a t which substances penetrate brain tissue through the blood-brain barrier is inversely related to their size and directly related to their lipid solubility; therefore, attempts have been made to design small technetium complexes with hydrophobic ligands. Three examples of neutral technetium complexes (YYmTc-BAT, Y9"'Tc-GTS, and 9ymTcPnAO) that have been designed topenetrate the blood-brain barrier are illustrated in Figure 6. Each of the agents readily crosses the blood-brain barrier, hut subsequently clears the brain tissue in a matter of minutes due to the lack of affinity for any particular receptor sites (14-16). By increasing the size of alkyl groups R1and Rz on the g9"'Tc-BAT, investigators have demonstrated that the rate of uptake in the hrain can be increased (14). Plasma protein binding also increases along with the lipid solubility of these complexes. The current challenge in hrain imaging is t o develop technetium complexes that are readily taken up by normal brain tissue
and remain long enough to allow SPECT imaging to he performed. Kidney Imaging The kidney is the principal organ responsible for the excretion of water-soluble substances from the body. T o some degree, all technetium radiopharmaceuticals will be excreted by the kidneys and eliminated in the urine. The pattern and amount of such renal excretion can be used to evaluate renal function. The functional unit of the kidnev is called the n e ~ h r o n (Fig. 7 ) . It is composed of a filtering apparatus referred to as the slomerul~~s. which is encapsulated in a strurture called the g o m a n ' s capsule. ~ollo&ngthe glomerulus is a multisegmented tubule system which reabsorhs nutrients and secrets unwanted substances. The arterial blood enters a kidney and passes through the glomerulus, allowing water-soluhle, low-molecular-weight species to undergo ultrafiltration. Molecular size, shape, and charge determine glomerular permeability. The basement memhrane of the glomerulus, which makes up the filter structure, consists of pores with effective diameters of ao~roximatelv20 A. The pores are lined with anionic charges U7), t h u s i t is easier for neutral and ~ositivelvcharaedsubstances topass than those that are negatively charged. Because plasma proteins are much high negative -areater than 20 A in diameter and carry~. chnrges, they will not crosi the glomerulus membrane structure. Dynamic imaging of the glomerular filtration proress is normnllv carried out withmmTc-DTPA.technetiu~n-diethvlenetriakne pentaacetic acid. The yymTc-DTPAreadiiy nasses throueh the elomerular filtration amaratus: and. be.. cause it is not reabsurhed into the blwd on passing thrnugh the tuhules. it flows into the urine. The rate d ~ a s s a of ~ the e QgmTc-DTPAthrough the kidneys becomes measure of renal clearance. Substances that cannot pass through the glomerular filtration membrane remain in the blood but can he eliminated in the urine by active secretion into the renal tubules (Fig. 7) (18). Uptake of 99mTcradiopharmaceuticals in the renal tubule provides a means of statically imaging the kidneys. The complexes used to image static tubular fixation are indicated in Figure 7. The prolonged retention of the static imaging agents is due to high binding affinities for sites in either the proximal or distal segments of the renal tubules (19). Binding sites in the renal tubules include thiol and disulfide moieties of various proteins. One such heavy metal binding protein, metallothionein, contains 50 mercapto groups per mole (20). Characteristics of the static kidney imagers include high binding site affinities, plasma protein binding, and low liver uptakes. Efforts have been made to design 99mTcradiopharmaceuticals that will image the dynamic function of renal tubule secretion. These complexes must have a particularly high extraction efficiency for some active transport protein but must not he retained for prolonged periods, as are the static imagers. The anionic organic acid active transport renal secretion process described by Despopoulos (21) has been exploited. In this regard, Davison and coworkers (22) have complexed technetium with N,N-his (mercaptoacetamido)ethylenediamine (DADS) to produce a complex, y9mTcDADS, that exhibits active secretion by the renal tubules. Although the agent is not as effective as 1311o-iodohippuric acid. normallv used to image renal tubular secretion, it shows promis"e for the deveropment of technetium radiopharmaceuticals for the imaging of active renal functions.
a
Figure 6 . Techmudm complexes deo gned tor SPECT main imaging: BAT = o r-ammoemanethiol:GTS = gluco~ls-~nlosemicaroaone; PnAO = propylene
amine mime
Bowman's c a p s u l e 'ma1 tubule
Figure 7. Schematic of a kidney nephron. GHA dimercaptosuccinicacid.
968
glucoheptonale; DMSA
Journal of Chemical Education
=
Imaging Liver Function Technetium rad~opharmaceuticalsdesigned to imnge liver t'unrtions fall into both the technetium-taraed -- and technetium-essential classes. In the technetium-tagged class, the 99mTc-sulfurcolloids are used to,evaluate the ability of the
reticuloendothelial system to remove unwanted particulates from the blood stream. Among the technetium-essential class are the 99mTc hepatobiliary agents, used to visualize functional pathways by which the liver excretes substances into the hile, which subsequently flows into the intestines. Typically these agents might be used to differentiate hetween patients exhibiting similar symptoms arising from either a cellular disease (such as hepatitis), or a simple hile duct obstruction. T o image the dynamic process of hepatohiliary excretion the radiopharmaceutical must proceed through the system as a bolus. Ideally, this requires that the radiopharmaceutical he eliminated from the blood by only one route, in this case through the bile. Diagnostic imaging of liver hepatobiliary function was first performed with 1311-labeledrose bengal in 1955. The rose hengal was not an ideal choice because of its sustained retention in the liver and blood. The use of technetium in hepatobiliary imaging was introduced in 1972 with the use of technetium-99m penicillamine. Since then, a wide variety of technetium complexes have been designed for this purpose. The development of these agents is discussed in detail in an excellent review by Chervu, Nunn, and Loberg (23). In designing a radiopharmaceutical to image hepatobiliary function, one must attempt to minimize excretion by the kidneys and maximize extraction by the liver. Empirical structure-activity relationships have been established that enable one to predict the degree of liver uptake and extent of kidney excretion exhibited by a prospective hepatobiliary agent. The physical properties determining the favored excretory pathway of an agent are its size, charge, protein binding, and functional group hydrophobicity. The more lipophilic species tend to be excreted by the liver and the hydrophilic species by the kidneys (24). The liver is highly vascular and receives over 20% of cardiac output. If extraction by the liver is high, i t follows that the imaging agent will clear the blood rapidly. The excretory pathway involves passage of the agents from the blood, on one side, into well-ordered monolayers of liver cells (called hepatocytes), followed by secretion on the other side of the cells into channels that lead to the bile ducts (Fig. 8). Species cross the hepatocyte membranes by independent active transport processes specifically designed for either anions, cations, neutral substrates, or bile salts (25,26). The rate of hepatohiliary excretion is governed by the interaction with these carrier-mediated active transport systems. Once inside the hepatocytes, the species can either be bound to some intracellular protein, metabolized, or secreted into the hile. Two general types of ligands have been combined with technetium to form hepatobiliary imaging agents. These include the iminodiacetic acid complexes, 9gmTc-IDA(27,28) and the pyridoxylideneaminate ~omplexes,~~'"Tc-PDA (29). The latter are composed of Schiff bases t o which various hydropbobicaminoacids are attached. Structuralconfigurations of these complexes are illustrated in Figure 9. Although certain reaction conditions can result in the production of multiple complexes, the active complexes are believed to have technetium in the +3 oxidation state, bestowing a -1 charge on the complexes. This means that the uptake in hepatocytes is determined primarily by the anionic active transport process and by the way the complexes fit into the carrier proteins of that system. By substituting different functional groups onto the aryl moieties of the ligands, the hepatic uptake and kidney excretion can be altered (27). Specifically, adding larger alkyl groups in the ortho position of the aromatic ring of the IDA complexes (Fig. 9) increases the hepatic uptake and decreases the urinary excretion. Similarly, making pyridoxylideneaminate complexes with dimethyl-substituted phenylalanine, as opposed to halo-substituted derivatives (Fig. 9), creates complexes with faster hepatobiliary clearance (29).
Generally, the more hydrophilic these complexes become, the ereater tendencv . thev. have to be excreted bv the kidneys. As a result, the organiclaqueous partition coefficients of the lieands mav he correlated directlv with h e ~ a t i cclearantes of the corresponding complexes. Also, the molecular weight-to-charge ratio of each complex can provide an indicator of hepatobiliary excretion, where the ln(mo1 w t l charge) is directly proportional to the net hepatobiliary excretion (Fig. 10) (9,27).
-
-
Bone lrnaglng
In the early 1970's, a revolution took place in nuclear medicine when it was discovered that technetium-99m phos-
Figure 6. Crosssection of a normal liver lobule: (A) central vein. (6)sinusoids carrying blood to central vein, (C) sinusoid endolhelial cells, (0) canaliculi carrying bile to bile duct. (E) bile duct. IF) hepatic artery. (0)portal vein, (H) hepatocyte. (Redrawn and adapted t o m Ham. A. W. "Histoiogy." 6th ed.. Lippincan. Philadelphia. 1969.)
Figure 9. Structures of ~ Tcomplexes c used to image hapatobiliav excretion.
Volume 62
Number 11 November 1985
969
Table 1.
Dlphosphonate Ligands
Table 3.
Uptake as a Function of Dlphosphonate Chaln Length'
0 R, 0
II I II I I I
HO-P-C-P-OH
OH RI OH ~p
% kselg methylene dlphosphonata (MDP) hydroxymethylene diphosphonate (HMDP) N-methylamino memylene diphasphonate (NMMOP) N,Ndimethyiamino methylene diphosphonate (DMAD) 2.3-dicarboxypropyl diphosphonate (OPD) hydroxyethylidene diphosphonate (HEDP) 3-amino-1-hydroxypropyl diphosphonate (APO)
R1
RI
H OH OH NCH3
H H H H
N(CHd2
H
CHCOOH CH&OOH OH
H
OH
CHS CHICHINHI
Compound
n
Bone
Blocd
technetium-99m pyrophosphate would localize in bone, but the hydrolysis of the pyrophosphate produced complications in blood clearance and soft tissue uptake. Shortly thereafter. it was found that "mTc bone-imaeine aeents ~" ~ ~ could be produced with polyphosphates and hydroxyethylidene diphos~honate(HEDP). The HEDP was known to inhibit bonegrowth, thus its interaction with the hydroxyapatite mineralization process had been established. The technetium diphosphonates proved to he the agents of choice because they were not metabolized, thus giving suitable hlood and soft tissue clearances. Since the discovery of the bone agents, a variety of diphosphonate ligands have been utilized to make the complexes, some of which are listed in Table 1.TheBmTc-MDPhas proved to he the most popular bone scanning agent because of its rapid blood clearance. Although the exact mechanism by which technetium-99m phosphates are taken up in bone continues to be dehated, evidence suggests that the comolexes chemisorb onto the surface of &ly formed crystailine bone (31). Empirical studies with the technetium-99m ~ o l v ~ h o ~ ~ hhave a t edems onstrated that the longer the chain ieigthof the polyphosphate (n = 2 to 30), the lower the uptake of the technetium complex in hone (32, 33) (Table 2). Similar studies with several polymethylene diphosphonates reveal that the smaller the diphosphonate ligand, the higher the uptake in bone and the lower the uptake in muscle and hlood (34,35) (Table 3). Since imaging contrast between bone and the background is desired, the bone-to-soft tissue and bone-toblood uptake ratios become critical measures of imaging performance.
-
~
Table 2.
Uptake as a Function of Polyphosphate Chaln Length
Chain Length (n)
27-30
%DOse/TotaI Skeleton Schumichene Kingb
...
8.4
'Schumichen. C., and Nakken. K . F.. Nml. Med (Stung). 13, 139 (1974). Wing. A. G.. Christy. 8.. Hupt. H. 8.. etal.. J N w l ~ e d14. . 895 (1973).
phate complexes imaged hone (30).Todav bone s c a n n i n ~is the most widely usedimaging procedure,accounting for the diagnoses of millions of bone disorders each year. The mature mineralized matrix of the bone, called cortical bone, consists primarily of crystallized hydroxyapatite, CaI O ( P O M O H ) The ~ . source of . nhosnhate for the erowth of these cr~stalscomesfrom the enzymatic hydrolysi of phosphate esters and pyrophosphate. I t was first discovered that
~
.
Figure 10. Hepatobiliary excretion of 'DToHIDA analogs versus ln[mol. wt/charge].(Reprint& with permission from Burns. Marzilli. Sowa, and Wagner, Reference (2n.)
970
Journal of Chemical Education
Figure 11. Cross-section 01 bone wall: (A) blaad vessels. (8) calcified bone matrix, (C) bone fluid, (Dl ~teoprogenitorcells. (E) perlvascular fluid. (F) osteoclast. (G)osteocyte. (H) osteoblasts. (I) periosteum. (J) muscle. (K) marrow cavity. (Redrawn and adapted fmm Owen, M., Howlen. C. R.. and Trittln. J. T., Cakil. Tissue Res.. 23, 37 (19773.)
-
Table 4.
Uptakes ot Various Technetlum-99m Dlphosphonate Complexesa
noce .- ----
%
Complex 99Tc-HMDP g9Tc-DPD OSTc-NMMDP "Tc-ADP *9Te-MDP "TC-DM AD
whole skeleton
Normal Bone Muscle
New Bone Normal Bone
58
435 260 488 407 395 265
3.43 3.27 4.35 3.79 4.48 6.56
52 31
31 30 19
Table 5.
Characterlstlcs of 9sTc-HEDPComplexes Separated by HPLC
9sT~-HEDP Component Charge
Volume mL/mol
Normal Bone Uptake %Dl9
New Bone Uptake %Dig
'Subrammian, G.. el al.. RsdloIogy, 148,823 (1983). In normal bone tissue technetium-99m diphosphonates are believed to accumulate (as seen in Fig. 11) at an interface between osteoid tissue, among the osteoprogenitor cells (D), and calcified bone matrix (B), in the perivascular fluid (E) where active mineralization is ongoing (3637). The osteoid is osseous tissue tbat has not yet calcified. The cells within the bone that are involved with bone formation and resorption (i.e., dissolution of old bone) are the osteoblasts, osteocytes, and osteoclasts (Fig. 11). Osteoblasts (H) are the boneforming cells that secrete substances necessarv for hone minernkation. osteocytes ( G )are bone cells surkounded by the calcified bone matrix. The osteocvtes aid in the transport of substances between cells withiithe hone matrix. The multinuclear osteoclasts (F) are cells that erode and resorb old bone. When bone is resorbed, the osteoclasts recycle substances by passage to the osteohlasts via the osteocytes (Fig. 11). In normal bone, the osteoid interface where the technetium-99m diohosnhonates accumulate is adiacent to the marrow cavity in ;he perivascular fluid, denoted by region E in Figure 11. The formation of new hydroxyapatite crystals begins in small matrix vesicles in the perivascular fluid (38) (Fie. 12). Small hvdroxva~atitecrvstals grow until thev break from the ves~clesakfhecome kxposei to the p e r i v i cular fluid. The technetium-99m d i ~ h o s ~ h o n a t have e s easv access to the perivascular fluid by diffusion from the blood capillaries (A) through the slit-pores between the epithelial cells of the capillary walls (Figs. 2 and 11). I t is suspected that the technetium-99m diphosphonate complexes accumulate on or are incorporated within these actively growing
hvdroxva~atitecrvstals. Because technetium-99m d i ~ h o s phona
