Radiopharmaceuticals for renal studies

Radiopharmaceuticals for Renal Studies L. Rao Chervu, Leonard M. Freeman, and M. Donald Blaufox A review of the wide spectrum of radiopharmaceuticals developed over the last 15 yr for application in the evaluation of diseases of the kidney and urinary tract is presented below. The radiolabeled contrast agents including Hippuran have been extensively applied for renal function assessment. The kinetics of clearance of many of these agents are not yet satisfactorily established, and there is no ideal agent for routine clinical application, particularly by external counting methods. Many agents for intrarenal blood flow measurement have been reported that have yet to be adapted for obtaining clinical information. Renal morphology has been studied using several radiopharmaceuticals that are either fixed in the renal

tubules for a sufficiently long time (chlormerodrin,VV"Tc-Fe ascorbic acid) or cleared from the kidneys fairly rapidly (Hippuran). These agents suffer from several disadvantages because of suboptimum energy of the radiolabel for use with imaging equipment, or relatively high radiation dose, o r slow rate of excretion with inadequate organ specificity. It is hoped that the development of new radiopharmaceuticals tagged with radionuclides that have ideal imaging characteristics (~mTc or tliln) and satisfactory renal clearances will provide a major breakthrough in the continuing search for satisfactory renal agents for imaging and function tests in the near future.

HE constant composition of the blood and the extracellular fluids depends to a large extent on the functional integrity of the kidney. The combined blood flow of both kidneys approximates 20 to 25% of the cardiac output, a blood supply far in excess of its own metabolic needs. The body depends principally on the kidney to maintain a constant osmolarity, concentration of the principal cations and anions, and constant pH, as well as to excrete a number of products of metabolism. Noninvasive function tests of renal perfusion and excretion using radioisotope methods are extremely valuable alternatives to constant infusion and/or catheterization procedures that are routinely used for evaluation of the status of the genitourinary system. The information that is generally useful for diagnosis or management when combined with the concurrent measurements of blood and plasma volumes, solute clearances, and total urine flow includes: (a) total renal blood flow, (b) total effective renal plasma flow, (c) glomerular filtration rate, (d) morphology of the kidney, (e) postvoiding residual urine volume, ( f ) ureterovesical reflux, and (g) general evaluation of the integrity of the urinary collecting system. The radionuclide techniques for the evaluation of diseases of the urinary tract have come increasingly to be accepted as routine diagnostic procedures exclusively or in conjunction with the radiographic procedures for identifying renal size, shape, and functional integrity for evaluation of surgically correct-

T

Front the Department of Radiology, Albert Einstein College of Medicine, Bronx, N. Y. L. Rao Chervu, Ph.D.: Assistant Professor of Radiology; Leonard M. Freeman, M.D.: Associate Professor of Radiology: M. Donald Blaufox, b,l.D.: Associate Professor of Medicine and Radiology, Department of Radiology, Albert Einstein College of Medicine, Bronx, N. Y. Supportedin part b)' NIH grant NLAII984, USPllS grant 5 POIAMI4877, USPttS contract 86-67-300, and James Picker Foundation RG 72-6. © 1974 by Grune & Stratton. Inc. Seminars in Nuclear Medicine, Vol. 4, No. 1 (January),1974

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CHERVU, FREEMAN, AND BLAUFOX

able disorders of renal perfusion and urodynamics and in renal transplant cases for early evaluation of vascular patency of the anastomoses including differentiation of progressive onset of rejection from acute tubular necrosis and other sources of reduced renal function. The use of radiorenography for the assessment of qualitative and quantitative aspects of the above functional studies using radionuclides has continued to be modified during the last de6ade owing to a parallel contribution from two major fields of development. Radiopharmaceuticals incorporating short-lived and high-photon-yielding nuclides have increasingly been made readily available. The reduced radiation dose to the patient, especially in pediatric cases, which is of greatest concern in any of these radionuclide studies, has permitted serial evaluation wherever necessary within reasonably short intervals with minimal discomfort and risk to the patient. High-resolution camera imaging devices coupled with peripheral dedicated or on-line data processing equipment have been developed with which semiquantitation can be achieved with great precision and speed, particularly in dynamic studies. The present survey is an attempt to review succinctly the gamut of radiopharmaceuticals developed over the course of the last 15 yr with a critical assessment of the problems in application of these preparations for routine clinical use. The need for improved radiopharmaceuticals, particularly for quantitative renal function studies, is placed in perspective here. Physiologically inert compounds when introduced into the blood stream may undergo excretion mainly by two different pathways, either via the hepatobiliary system or via the urinary tract. The extent to which one organ excretory route predominates over the other depends upon various factors, among which are the strength of the protein binding of the particular compound and the functional status of the principal organ. Under these circumstances it is essential to examine the different classes of radiopharmaceuticals of interest from the viewpoint of their mechanism of renal handling, which is briefly discussed below. Three discrete renal processes are known to be involved in the elaboration of urine: glomerular filtration, tubular reabsorption, and tubular secretion. Glomerular filtration is a physical process in which the hydrostatic pressure at the glomerular capillary level yields a net positive pressure to overcome frictional resistance in the membrane and colloid oncotic pressure resulting in a protein-free filtrate of plasma entering Bowman's space. Other than that, no measurable work in the formation of the filtrate or any component of it is performed by the cells of the filtering surface. The uitrafiltrate formed through this process contains all the diffusible substances in the same concentration as they occur in the plasma. Diffusibility requires dimensions below a certain size consistent with the pore dimensions of the membrane and subject to GibbsDonnan distribution in the case of the electrolytes. The passive process of glomerular filtration may be quantitated by the measurement of the rate of renal clearance of a particular substance in plasma, provided the same substance meets certain criteria that will be discussed later. The tubular reabsorption mechanism transports a number of solutes present in the glomerular filtrate from the tubular lumen into the pertibular capillary

RADIOPHARMACEUI"ICALS

FOR RENAL STUDIES

5

fluid. Very discrete active or passive mechanisms that are sometimes interdependent are involved in the reabsorption of various components of the filtrate. Several reabsorption mechanisms are located in the proximal segments of the renal tubules. Some, such as the transport mechanism for glucose, are remarkably efficient in operation. The tubular secretory process involves the transport of materials from the pertibular fluid to tubular lumen. The factors involved in this process include active secretory mechanisms that exhibit an absolute limitation of transport capacity and passive secretory mechanisms involving diffusion of materials down concentration gradients or gradients of electric potential. These mechanisms adequately explain the tubular secretion of organic acids, strong organic bases, weak acids and bases, and various other organic compounds. The active site of secretion of many of these compounds is the proximal tubule, and a continuous supply of energy is necessary to move the transported materials from blood to urine either directly or indirectly. The energy-requiring systems are susceptible to metabolic inhibition. It is essential that the detailed mechanism of renal handling of any agent by the process of glomerular filtration and/or tubular reabsorption and secretion be established for consideration of application for renal function studies. AGENTS FOR THE MEASUREMENT OF GLOMERULAR FILTRATION RATE

The measurement of the giomerular filtration rate (GFR) to follow the evolution of renal disease has been stressed by many workers long before the availability of radioactive tracers. These measurements may be performed in man using a wide variety of nonradioactive substances such as inulin, hyposulfite, mannitol, or creatinine. The criteria for adapting any agent to the measurement of the filtration rate are: (a) the substance must be freely filterable through the glomerular capillary membranes and not be bound to the plasma proteins nor sieved in the ultrafiltration process; (b) it must be physiologically inert and not be metabolized by the kidney; (c) it must be neither reabsorbed nor secreted by the renal tubules; (d) it must be nontoxic and must not exert any effect on renal function when infused in quantities varying over a wide range of concentrations; and (e) it must be measurable with a high degree of accuracy in both plasma and urine in a routine laboratory procedure. The clearance of inulin to measure G F R was proposed by Richards et al. ~ and Shannon and Smith 2 in 1935. The clinical use of inulin clearance as a measure of the giomerular filtration rate has generally been accepted as a standard for these measurements. Endogenous creatinine a n d urea clearances have also been measured, but both these compounds are known to be partially excreted by the kidney tubules. 3 This is unfortunate, since the ease of using an endogenous compound is obvious. The inulin molecule seems to satisfy all of the criteria mentioned above for the measurement of GFR, as proved by ample experimentation in a number of animal species and also in man. + The inulin preparation is administered in a concentrated priming dose in order to raise the plasma concentration to 10 to 20 mg/100 ml and then is infused at such a rate as to maintain the plasma level constant. The blood samples and urine samples (sometimes obtained by catheterization) are accurately determined for their

6

CHERVU, FREEMAN, AND BLAUFOX

inulin content by chemical analysis. The accurate chemical analysis techniques are quite demanding, and in this respect radiochemical methods are more precise and more routinely adaptable. The relevant inconveniences involved in using the nonradioactive substances and bladder catheterization for the measurement of such an important parameter as G F R in the study of renal function and evaluation of patients with renal disease have encouraged the search for radioactive compounds that have the same renal excretory behavior as inulin. In the main, the agents that have been investigated are labeled inulin and its derivatives, radioiodinated urological contrast agents (Hypaque, Renografin, Conray, etc.), radiocyanocobalamin (vitamin Bt2), and the metal chelates. The radiochemicai stability and the extent of renal handling of the radionuclide preparations are two additional parameters to be taken into consideration for application of radiopharmaceuticals as G F R agents. Inulin and Derivatives

Inulin labeled with 14C (at the carboxyl group) has been recommended as a G F R agent by Cotlove. s This preparation did not'gain widespread clinical use owing to the fact that the beta activity of this isotope requires the use of a liquid scintillation counter for measurement of samples and also does not permit external counting. The inulin carboxyl ~4C has recently been reported to differ from inactive inulin in its physiologic behavior, 6 and there may be a highly significant deviation of the clearance of 14C inulin from that of native inulin. Hydroxymethyl 14C inulin containing no other functional group other than those present in native inulin, on the other hand, has been shown to have identical clearance behavior as that of native inulin, and a high degree of correlation with native inulin is obtained in the measurement of GFR. 7 The labeling of inulin with 131I at a primary hydroxyl group has not been achieved, but Brooks et al) have succeeded in introducing an allyl ether group into the inulin molecule followed by the addition of the radioiodine at the double bond. According to Concannon et al., 9 in the fresh 125I allyl inulin preparations, a portion of activity is loosely bound to the inulin molecule and with time dissociates from the molecule. Satisfactory preparations are obtained with purification from free iodine by repeated passage of the aged preparation through anion exchange columns. A high degree of correlation between the chemical inulin method and the 125I allyl inulin method for the determination of G F R has been reported, 1° provided the impurities are removed prior to each use.

Several reports on the determination of G F R using m1 chloroiodopropyl inulin have been published, tH4 and they indicate that the clearance values based on the isotope determinations seem on the average to be slightly lower than the chemical values simultaneously determined, though a significant correlation exists between the two series of measurements. Tubis et al) 5 have reported yet another preparation of radioiodinated inulin, propargyl inulin t3tI, which remains fairly stable for a reasonable length of time with about 3 ~ radioiodine released after 16 days of storage. It must, however, be pointed out that the gradual liberation of free iodine in the preparation and the dissociation of the iodine in vivo after the injection (as evidenced by the thyroid uptake) are

RADIOPHARMACEUTICALS FOR RENAl. STUDIES

7

two important factors that might lead to the lowering of the GFR value owing to the markedly lower rate of renal clearance of the inorganic iodine. 16 Evidence has not to this date been presented to show that the iodinated inulin has different biological behavior than the inactive inulin. The preparation of 5'Cr inulin has been reported by Johnson et ai., ~7 and this agent has been used for determination of GFR in dog and in man) 8:9 The preparation is simple and it is reported to be stable in vitro and in vivo. This warrants further investigation in terms of its total clearance pathway and compartmental distribution. Unfortunately S~Cr is far from the ideal nuclide, which would preclude any widespread application of the compound.

Vitamhl BI2 Labeled with 57C0 or SSCo, eyanoeobalamin and hydroxocobalamin are eliminated by the kidney essentially by means of glomerular filtration similar to the renal clearance of inactive vitamin B~2. This was first reported in studies in rabbit :° and in man by Watkins et al. 2~ The application of this agent for the measurement of GFR is based on the following grounds. Vitamin Bl2 is bound to serum proteins transcobalamin I and transcobalamin II.22"23The endogenous B~2 is assumed to be bound to the former, which in normal serum is almost saturated. The radioactive B~2 added in vitro or in vivo is recovered in the transcobalamin I1 fraction. In vivo, the transcobalamin II fraction acts as a transport protein, and the tissue sites, especially the liver, remove the vitamin with constant increase in the amount bound to TC I. When large amounts of stable vitamin B~2 are administered, the binding capacity of the tissue and the plasma binding sites in vivo is exceeded, and the circulating, free, nonproteinbound vitamin will be excreted purely by glomerular filtration. The radioactive vitamin Bj2 given shortly thereafter will be largely unbound, and its clearance is a measure of the GFR. The renal clearance of vitamin B~2 labeled with STCo or SSCo is compared with that of inactive inulin by a number of authors. 24-32 The presaturation of serum is carried out using cyanocobalamin or hydroxocobalamin, the latter being more firmly bound to the plasma proteins. 33'34 The clearance values obtained using the above methods are found always to be lower than the inulin clearance values, the ratio of U/P total B~2 to U/P inulin ranging from 0.43 to 0.99. The main reason for this wide discrepancy has been attributed to the variability of the binding of the labeled agent to the plasma proteins owing to the partial exchange of radioactive B~2 with inactive vitamin during the clearance procedure, 29'32'35 even in the same patient under different conditions. This can be corrected to some extent by separating the bound and free radioactive vitamin B~2 in each plasma sample using a complex and tedious procedure. Too many variables, such as temperature of incubating mixture, temperature of the system during the measurement of binding, and plasma dilution, influence the determination.32 The multicompartmental distribution of the cyanocobalamin also does not permit the estimatibn of G F R via the single-injection technique with significant uptake ofSTCo B~2 in liver, 2s besides other major tissue and body fluid distributions. Thus, though the renal clearance of unbound radioactive B~2 is practically the same as that of inulin, the 57C0 Bz2 procedure as a replacement

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CHERVU, FREEMAN, AND BLAUFOX

for the measurement of GFR will have to await a more detailed investigation of the binding and distribution of Bt2 in various body compartments. It is doubtful that this agent would prove useful as a clinical routine procedure.

Labeled Urological Contrast Agents lodinated benzoic acid derivatives with more than two or three atoms of iodine per molecule have been used extensively as contrast media for intravenous urography. Since iodine is a heavy atom with a high x-ray absorption, they are particularly well suited. Kidneys do not handle these contrast media similarly with respect to the degree of glomerular filtration, tubular reabsorption, or tubular secretion, and their extrarenal pathways differ considerably. Among these, the agents that have been labled with ~25I or ml and investigated for renal function studies (GFR) are illustrated in the general chemical structure shown in Fig. 1, with the substituents of the various agents shown in Table 1. Proximal tubular secretion at lower plasma levels and a self-inhibiting effect on the proximal tubular secretion at higher plasma levels have been reported in studies with Urokon. 36 Also, considerable binding of this agent to serum protein 37 precluded its use for further renal function studies. Of the remaining three agents, diatrizoate and iothalamate, which have similar structure, are more extensively studied for renal function testing. Diatrizoate. The labeled compound is commercially available in the form of a neutral aqueous solution of the sodium salt (Hypaque) or the methyl glucamine salt (Renografin) labeled with either mI or 13'I. The first attempts to study renal function using mI Hypaque were described by Kimbei and Borner) g Winter and Taplin 39 studied the application of this tracer for renography. Its renal clearance and body distribution in animals and in man formed the subject of intensive investigationf1-52 As is common with a large number of organic compounds labeled with radioiodine, the iodine atom tends to split off from the labeled Hypaque gradually upon storage owing to autoradiolysis, the amount of inorganic iodine released being dependent upon the specific activity of the preparation) 6 The presence of the free iodine is reflected in the lower clearance values5~ and in thyroid uptake, though to a small extent,46 which points to the necessity of control of radiopharmaceutical purity in the preparations. Plasma protein binding of this agent is lowY '52 Evidence has also been presented to exclude any tubular handling of the tracer. Tubular reabsorption is ruled out on the basis of constancy in clearance values with wide variations of urine flow.48Tubular secretion is similarly invalidated with tubular blockade COOH

I

I

R2

RI I

Fig. 1. General structural formula of iodinated contrast agents.

RADIOPHARMACEUTICALS

FOR RENAL STUDIES

Table 1. Substituent Groups in Different Urological Contrast Agents Generic Name Urokon, Triopac M.iokon Hypaque, Renografin Conray

Compound

R1 (Position 3)

R2 (Position 5)

acetrizoate diprotrizoate diatrizoate iothalamate

H NHCOCH2CH 3 NHCOCH 3 NHCOCH3

NHCOCH 3 NHCOCH2CH 3 NHCOCH 3 CONHCH3

experiments48 and on grounds of constancy of clearance values with increasing plasma loading of the stable urographic agent or PAH loading. 44's3 These considerations lead to the conclusion that the clearance of the diatrizoate (Hypaque 125Ior m I ) c a n be used for the measurement of G F R in man by the constant infusion procedure employing the UV/P relationship for renal clearance. A very significant correlation between Hypaque or Renografin clearance values and those of inulin has been reported, with a mean clearance ratio of unity in normal and pathological conditions in adults and in children. Detailed studies on the total clearance of radioiodinated Hypaque have shown that the extrarenal pathway for the agent becomes more important with increasing reduction of GFR. 54'5s lothalamate. This agent has a similar chemical composition and exhibits quite close renal clearance properties to diatrizoate. The ~2SI-labeled agent is commercially available in pure form with minimal contamination of free iodide (<270) even after storage up to 60 days. 56The plasma binding also is reported to be less than 370 in man; 56 Maher and Tauxe 52 report a much greater plasma binding (8 to 2770) using ultrafiltration techniques. The nature of the binding, its renal handling, and its dependence on carrier doses have not been investigated in detail. Tubular excretion of the tracer is absent, as indicated in experiments with dogs s7 and judging from the constancy of clearance values at high and low plasma concentrations of the unlabeled agent in man. 58The renal clearance of radioiodinated iothalamate has been found to be in close agreement with that of inulin in animals 59 and in a large number of patient series in adults 56'58'6°-~ and in children. 65"66Extrarenal elimination has been reported for the tracer in an anephric rat. 67 The characterization of distribution and elimination of the tracer in adult man in different stages of renal disease is awaited.

Metal Chelates The metabolism of ~4C-labeled ethylenediaminetetraacetic acid (EDTA) administered parenterally in the rat and in man has been studied by Foreman et al. 68"69They observed that the agent is eliminated essentially unchanged by G F R and tubular secretion to the extent of 95 to 9870 in about 6 hr. Heller and Vostal 7° measured clearances of EDTA in rats and found them to be much higher than that of inulin. Interest in the clearances of metal chelates has been revived owing to the strong complexing properties of EDTA and its analogs with a number of metals and the possibility of their application in case of poisoning by radioactive and stable metals. The in vitro stability constants of these complexes are high; however, their stabilities may be affected in vivo. Chelates that enter into a cellular space may be exposed to an increased mass of binding agents and to systems with extremely high affinities for specific metals.

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CHI:RVU, FREEMAN, AND BLAUFOX

Thus in vivo the chelates may be bound t o different organs (e.g., ~°3Hg EDTA is detectable in the kidneys for several days). A large number of physiologically stable metal chelates tagged to radionuclides have been proposed as renal agents during the last few years, of which 5tCr EDTA, 99mTc DTPA, 14°La DTPA, t69yb DTPA, and H3mln DTPA received somewhat greater attention for application for G F R measurements.7'-75 Several other complexes of EDTA, DTPA, and citrate with STCo, SSCo, 6SGa, 99mTc, rain, 113~In, 114rain, ItSmln, 169yb, and 197Hg may also be excreted by glomerular filtration and have been suggested for obtaining this parameter. 76-79The studies with these complexes are quite limited, and physical properties in many cases are not satisfactory for human application. Adequate validation of the true renal handling is still not available for many. SlCr EDTA. This complex was originally suggested by Myers et al. 8°'s~ for renography and was reinvestigated by Stacy and Thorburn 7~as a GFR agent. It has a high degree of radiochemical stability and high specific activity. Preparations (60 mCi/mg) are available in very pure form commercially. A very small fraction is bound to the serum proteins (< 2~/o).35"71'82To a small extent, red cell binding is considered likely,82'83but no quantitative measurements are available. Tubular secretion for this complex in dog and in sheep has not been demonstratedY '84 The extrarenal elimination of the tracer is insignificant,82"gs and whole-body retention of activity after several days is less than 1~o, as determined by whole-body counting.86 Somewhat larger retention of activity of S~Cr EDTA has been reported in the kidney after 72 hr. s5 It appears that these data at least partially support the concept that the labeled SICr EDTA is excreted by glomerular filtration. A high degree of correlation is obtained between this agent and the classical inulin clearance measured by both continuous and single-injection techniques. 82-85"sT'ssThe absolute value of the clearance measured with 5XCr EDTA is lower than that of inulin, to the extent of 5 to 20~o in animals and in man. The reason for the difference between the clearances is not known with certainty and awaits a more detailed study of the distribution of the complex. However, this shortcoming must be considered in applying this agent. DTPA Complexes. t4°La,113rain,and x69yb DTPA complexes4s'73-75"89-91 have been studied in some detail for the measurement of GFR by several workers. The complexes are eliminated by the kidney essentially by glomerular filtration. Clearances in animals have been determined in order to study the effects of increasing plasma concentration of the agent, different urine flow rates, and tubular blockade.4s No significant differences in the plasma clearance have been reported, t4°La DTPA results in a high radiation dose because of its associated beta decay, and hence it is not suitable for human application. Comparison of 169yb DTPA, ll3mln DTPA, and 14C inulin have been made in a large number of patient series by Sziklas et al.,9j and they report a high degree of correlation between the clearances obtained simultaneously. Gel filtration characteristics of the plasma and urinary radioactivity indicate that the complex is stable in vivo and the radioactivity is not bound to protein fraction.9° A reasonably stable preparation of 99mTc DTPA has been reported recently,92.9s and the evaluation of this agent for the measurement of GFR has been reported in

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animals and in a limited number of patient series. 7z'94 In dogs, using constantinfusion techniques, no change in 99raTc DTPA clearance was noted at different urine flow rates or tubular blockade. Slightly lower clearance values were obtained compared to iothalamate clearances in patients. About 107o of the activity is reported to be protein-bound. The renal handling of the agent under different degrees of renal impairment and the extrarenal pathways must be evaluated in greater detail, besides further study on the stability of the preparation itself. RENAL PLASMA FLOW AGENTS

The clearance of a tubular-secreted compound would give a measure of the renal plasma flow if the secretion were complete. The measurement of this parameter is based on the application of the Fick principle, which states that the rate of blood flow to the kidney is directly proportional to the rate of excretion and inversely proportional to the arteriovenous difference of the concentration of the agent in question. If the compound were totally extracted in a single pass during the course of perfusion of the kidney, the calculated clearance would give the total renal plasma flow. More realistically, the calculated clearance would be somewhat less than the total renal plasma flow, whence a measure of the effective renal plasma flow would be obtained. Paraaminohippuric acid (PAH) is eliminated by the kidneys through giomerular filtration (2070) and tubular secretion (8070). During lone passage through the normal kidney, about 9070 of the PAH is eliminated and the rest is returned to the circulation, and hence this compound has been chosen as the standard reference compound for measuring the effective renal plasma flow (ERPF). Many organic acids of diverse structure and properties are subject to active tubular secretion.9s Organic bases too are excreted in similar but separate mechanisms. Members of both classes of compounds undergo passive tubular reabsorption to a certain extent, which can partially be predicted on the basis of their physical properties and physiological variables, some of which are lipid solubility, pK,,, tubular fluid pH, and volume. Several organic acids and bases secreted by the renal tubules have been studied, and the chemical structural requirement has been examined in detail with no clear definition of substrate specificity.96 A compound that is rapidly excreted unchanged and as a consequence not extensively metabolized is ideally suited for the measurement of the effective renal plasma flow. Inactive iodopyracet was the first substance used for the measurement of ERPF, but owing to the complicated chemical method of estimation, it has given way to p-aminohippuric acid (PAH). While the chemical method of estimation of ERPF using PAH is precise, a radioisotopic method is preferable, particularly if external measurements can be made with rapid processing of clinical results. PAH molecular structure is such that it would not be possible to label it with a suitable gamma-emitting nuclide. Smith 4 suggested that OIH (orthoiodohippuric acid) might replace PAH for the measurement of the effective renal plasma flow. The preparation of mI-labeled OIH has added a valuable radiopharmaceutical for the measurement of ERPF. 97 Radioiodine-labeled iodopyracet has also been considered for similar studies,98"99 though the large component of extrarenal pathway is a

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CHERVU, FREEMAN, AND BLAUFOX

NH--CH2--COOH

H2N~CO-T

(a)

~ ~ - -

CO--NH-- CH2-- COOH (b)

O

[

CH2 --COOH (c)

Fig. 2. Structural formulas of (a) PAH, (b) O|H, and (c) iodopyracet (Diodrast).

major handicap. The chemical structural formulas of PAH, OIH, and iodopyracet are shown in Fig. 2.

lodopyracet: 3, 5-Diiodo-4-pyridone-l-acetic Acid This compound is used in the form of sodium or diethanolamine salt labeled with t25I or 131I. It was first used experimentally labeled with t31I for the examination of kidney function by Oeser and Billion.96 Billion and Schlungbaum 99 reported that clearance of tracer doses of iodopyracet 131I w a s consistently 20 to 25~ lower than that of PAH. Significant extrarenal activity ~°° precluded its use for renogram or single-injection studies. ~°~ Elwood et al. 6°'t°2 have reevaluated this agent for the measurement of ERPF with carrier iodopyracet administration and obtained clearance ratios of mI iodopyracet to PAH of 0.96 to 1.03, which is quite close to the values obtained by Chasis et al. 1°3 using inactive compounds. These results are also corroborated by Maher and Tauxe. 52 The exact role of the carrier in the elevation of the clearances is not known and awaits a more detailed understanding of the plasma binding of the compound and the renal handling of the bound form during the excretory process. The major disadvantage of Diodrast appears to result from the very high rate of biliary excretion.

Orthoiodohippurate (H ippuran ) The labeling of this agent is accompllshed by simple exchange reactions first described by Tubis et al.97 and further modified in detail by various workers. ~°4"1°5The common radiochemical impurity is inorganic iodide, the content of which increases during storage, influenced by various factors (light,

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13

temperature, specific activity, etc.). It is necessary to maintain adequate quality control of the preparations to check free-iodine content of each preparation at the time of use, as otherwise erroneous clearance values are obtained. The application of the compound as an indicator for ERPF was first made by Burbank et al. t°+ and by Schwartz and Madeloff) °7 who determined simultaneous renal clearances of Hippuran and PAH in man. The clearance values of OIH were lower than those of PAH, with mean clearance ratios of OIH/PAH being 0.85, and these data are in general corroborated by several workers in studies in animals and in man. Tabulations of these extensive data are given by Mailloux and Gagnon. I°s These studies are also extended to children.+6 Many reasons for the lower clearances have been advanced, and these are (a) presence of free radioiodine in the preparation, (b) plasma protein binding, and (c) differences in tubular transport. The literature data are somewhat conflicting in arriving at a plausible explanation for the discrepancy. However, a combination of all three factors in affecting the clearance values need not be ruled out. Differences in the plasma binding of the two agents seem to be largely responsible for the lower clearance value obtained using OIH. 52 A detailed compartmental analysis of the distribution and mode of excretion of radioiodinated Hippuran has been made by Blaufox et al. 1°9 It was reported that the injected dose is almost completely excreted from the body in the urine, with a negligible fraction in the enterohepatic cycle and in the red ceil. AGENTS FOR MEASUREMENT OF RENAL BLOOD FLOW

The renal plasma flow and renal hematocrit may be used to give an approximate measure of the renal blood flow using the relationship RBF = RPF (I/I-HCt). Detailed knowledge of the true rate of perfusion of the excretory and nonexcretory renal tissue and intrarenal distribution of blood flow is essential in understanding the physiologic basis of many forms of renal disease. The renal vascular resistance along with the glomerular filtration rate and the urinary sodium excretion are important indexes in renal diseases and in the evaluation of hypertensive patients. The techniques for the measurement of renal blood flow are broadly classified ll°°"t under three categories: (a) methods based on the application of nondiffusible intravascular indicators, mI-labeled albumin, 99mTcalbumin, and 32p_ or 51Cr-labeled erythrocytes; (b) methods based on the extraction of an indicator or a test substance from blood involving use of agents excreted largely by tubular secretion, PAH or Hippuran, or other agents like 86Rb or 42K, or radiolabeled microspheres and mI-labeled albumin macroaggregates; and (c) use of inert diffusible indicators SSKr and IS3Xe by gas washout techniques. Although a great deal of potentially useful physiologic information is obtained using these renal blood flow measurements in animals and in man, they have not achieved routine clinical application. These methods are reviewed in detail in this issue by Griinfeld et al. AGENTS FOR RENAL MORPHOLOGY AND PERFUSION STUDIES

Mercurial Diuretics

Detection of space-occupying renal lesions with radionuclides effectively commenced in 1960 when McAfee and Wagner m used 2°3Hg chlormerodrin as

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CHERVU, FREEMAN, AND BLAUFOX

a renal scanning agent. Mercurial diuretics are almost entirely bound to serum albumin in the blood stream and as such are not filtered at the glomeruli. They are extracted from the plasma by the renal tubules and specifically accumulated by the renal cortical tissue, and their concentration in the renal medulla is low. Their excretion is very slow compared to the iodinated contrast media, and hence these agents are particularly suited for kidney imaging with a rectilinear scanner. About 4570 of the dose is accumulated in the kidneys in the case of normal renal function at 2 hr, and only 167o remains at 24 hr. ll3 No more than 1470 is eliminated via the liver in patients with normal kidneys, l'3 The replacement of 2°SHg with t97Hg reduces the radiation dose to the kidney by a factor of 10, and 197Hg chlormerodrin has since been one of the primary renal imaging agents. The biologic half-life of the fraction that is trapped in the kidney after 24 hr is quite long. Its Mersalyl (Salygran), another mercurial diuretic, has also been suggested, It4 but its lower renal uptake and higher rate of extraction by the G.I. tract are not desirable characteristics. The gamma emission of t97Hg (77 keV) does not permit as good a resolution as 2°3Hg, and its poor tissue penetration is another serious disadvantage, although much more so for brain than for the more superficial kidney imaging. The dosimetric advantages of 197Hg have outweighed these disadvantages in terms of its actual clinical utilization.

Hippuran and Other lodinated Agents Radioiodinated Hippuran is not a useful agent for imaging the normal kidney with a rectilinear scanner, since the rate of excretion is too rapid. H5 ~3'I OIH has been used for renal scans, particularly in cases of renal insufficiency with a conventional rectilinear scanner owing to the prolonged transit times. H6,H7 With the development of camera imaging devices and fast data-acquisition systems, the problem of short transit time has largely been overcome. However, the high energy of the mI (364 keV) and the radiation dose delivered to the organ are significant drawbacks. The availability of ~2sI Hippuran and other mI-tagged agents in the near future is expected to enhance the scope of application of these agents.

994Tc-labeled Radiopharmaceuticals The radionuclide 99mTc has ideal physical characteristics for application in nuclear medicine by virtue of its short half-life (6 hr) and radiation characteristics (140 keV of 9070 abundance). The efficiency of the 140-keV photon makes it more suitable for the Anger camera (I/2-in. crystal), and it has better tissue penetration than the low-energy gamma emitters like 125I or 197Hg. The low radiation dose allows large amounts of activity to be administered if necessary within short time intervals for serial measurements. In the chemical form of 99mTcO4, its absolute concentration in the renal parenchyma is so low that imaging studies have rather poor resolution. 99roTe pertechnetate, however, is used extensively for kidney perfusion studies. The urinary excretion of pertechnetate is relatively slow, about 867o of the filtered activity being reabsorbed by the renal tubules, rig and hence it cannot be employed efficiently for renal function studies in this chemical form. 99mTc pertechnetate has also been used for detection of vesicoureteral reflux in children, ng'~'°

RADIOPHARMACEUTICALS FOR RENAL STUDIES

IS

Many 99roTe-labeled agents have been developed in recent years that are useful for renal imaging and perfusion studies. 99mTc-Fe ascorbic acid was first proposed by Harper et al. ~2' for renal scanning. About 8~ of the activity localized in the kidneys '22 in the distal convoluted tubules remains intrarenal for 4 to 20 hr, which permits a delayed scan long after the early excreted activity is cleared from the collecting system, t23 This radiopharmaceutical is also useful in delineating the kidneys even in the presence of high BUN levels, but it is not as reliable as Hippuran in this situation. A commercial preparation is available for this agent, and although the preparation contains DTPA, its behavior is more similar to that of99mTc-Fe ascorbic acid.94 The 99rnTc DTPA complex and its potential for scanning and functional assessment has already been mentioned.7~'93 A new renal scanning agent, 99raTe penicillamineacetazolamide, has been reported recently. ~z4'~25This is based on the reasoning that the 99mTcacetazolamide might behave like a molecule largely excreted by tubular secretion, and hence a higher target organ-to-background ratio would result in a short interval of time in patients with severe r~nal disease. The complex is prepared by reduction of pertechnetate with penicillamine followed by complexing with acetazolamide. However, the preparation is 97~ protein-bound and appears to be excreted very slowly by the kidneys. The in vivo distribution is very complex, since the agent distributes itself in several body compartments. Hiramatsu et al. 126have suggested that low-molecular-weight protei'ns labeled with radioactive tracers of iodine or 99mTc may prove useful as kidney agents, since these are filtered by the glomeruli and reabsorbed intact by the proximal tubule in the cortex. The localization of the polypeptide caseidin labeled with 99raTe in the renal cortex has been reported. ~27The rapid accumulation of a large portion of the activity in the renal cortex seems to support its application as a radionuclide carrier for renal studies. 128 Several other 99mTc-labeled agents are proposed for renal studies, ~z9 and these await more detailed study regarding stability of their preparation and biological distribution. For the most part, these agents are excreted at far too slow a rate to offer any real advance in this field. Chelates

The chelates suggested for the measurement of G F R are also useful for imaging, since they,permit visualization of early uptake, renal structure, and subsequent excretion of the activity into the renal pelvis. Multimillicurie amounts can be administered to provide the high photon flux needed to obtain good quality images with rapid scanning and acceptable radiation levels even with long-lived radionuclide activities.7s RADIATION DOSE

The main virtue of the radioisotopic procedure is the speed and accuracy with which it can be performed employing a noninvasive technique with minimal discomfort to the patient and relatively low radiation dose to adult as well as to pediatric patients. In the case of serial examinations, these factors assume a great degree of importance. The detailed calculations of the radiation dose received by the critical organs and other organs into which the agent is likely

CHERVU, FREEMAN, AND BLAUFOX

16

Table 2. Radiation Doses With Different Agents in Renal Function Tesls (Normal Man)

Nuclide 51Cr

Physical Half-Life"

Princ~pal Photon Energy(keV) and Intensity

27.8 days

320 (9%)

57Co ~mTc 99mTc ll3mjn 125I 1311

1311 14OLa t69yb

197Hg

RadiationDose(mrad//~Ci) WholeBody Kidney Bladder

Gonads

0.002

0.03

270 days 122 (87%) 6.04 hr 140 (90%) 6.04 hr 140 (90%) 104 min 393 (64%) 60 days 27-35 (142%) 8.04 days 364 (82%)

0.004 0.006 0.008 0.006 0.003 0.021

0.1 0.040 0.27

8.04 days 40.2 hr

0.07

51Cr EDTA 51Cr inulin S7Co vit. BI2 99mTc DTPA 99mTc-Fecomplex l_13mlnDTPA GFR agent GFR agent, inulin, or iothalamate Hippuran DTPA complex

0.01

DTPA complex

364 (82%) 329 (20%) 487 (40%) 31.8 days 177 (22%) 198 (35%) 65 hr 67-80 (91%)

0.013

Remarks

0.,55

0.06 0.01 0.014

0.39 0.4

0.07

35

0.05

7.8

0.16

0.010 (testis) 0.030 (ovary)

chlormerodrin

Note: In patients, depending on the degree of renal function impairment, the radiation dose received would vary, Selective data from references 122, 90, 86, 130, 93.

to distribute itself and the whole-body radiation dose are important factors to be considered for the final application of an agent for clinical studies. The kidney in particular cannot be considered to be a homogeneous organ for calculation of radiation dose, since the cortex, medulla, and collecting system have markedly different functions and concentrate radioactive substances in different degrees. Indeed, considerable heterogeneity exists among the individual nephrons. A knowledge of this changing concentration in different portions of the urinary tract is thus important. Further, the accumulation of the activity in the bladder in man contributes the largest radiation dose to the gonads and the functioning bone marrow of the pelvis and the lower spine. These detailed data are not available for most of the radiopharmaceutical preparations currently available and are urgently sought. The available data on the radiation dose employing a variety of renal agents are listed in Tables 2 and 3. It should be mentioned that these represent the values obtained in normal functioning kidneys with normal clearance values. SUMMARY

Procedures utilizing radionuclides have been accepted in clinical nephrology and urology as valuable noninvasive techniques for the evaluation of the urinary tract and for the diagnosis of renal disease. The determination of serial clearances using radiopharmaceuticals for the assessment of renal function has been established as a valuable alternative to the classical chemical procedures that are time-consuming and cumbersome for routine application. However, these radionuclide procedures are not yet thoroughly evaluated in different degrees of renal functional disorders. This is partly owing to the inherent complexity of the renal and extrarenal handling of the radiopharmaceutical by the body and the distribution of the agent in the different body compartments

RADIOPHARMACEUTICALS FOR RENAl. STUDIES

17

Table 3. Radiation Doses From Radiophormaceuticals in Renal Function Tests in Children RadiationDose(mrod//~Ci) Agent

Whole B o d y

Kidneys

Bladder

Gonads

99rnTc O4"

0.05

0.15

0.04

0.008

19?Hg chlormerodrin

0.16 0.07 0.02 0.01 0.006 0.11 0.05 0.03

68.1 39.0 0.03 0.02 0.015 0.22 0.14 0.10

0.26 0.15 0.10 1.7 0.9 0.65

0.09 0.10 0.09 0.23 0.22 0.26

1~'~1iothalamate

1311Hippuran

Remarks veslcoureteral reflux studies in children 5yr old 3 yr old 12 yr old 1 yr old 5 yr old 10 yr old 1 yr old 5 yr old 10 yr old

Note: Data from references 119, 120, 66, 131.

under different pathological conditions. A large volume of literature has been generated in the adaptation of these clearance procedures for clinical application. The agents sodium iothalamate and orthoiodohippurate for the measurement of G F R and ERPF, respectively, labeled with nsI or 131l, although acceptable, should be used with caution in routine clinical application, particularly in regard to the choice of methodology. They are not entirely convenient for external counting applications, and radiation dose could still be significantly lower. There is a need for greater effort directed toward introducing radiopharmaceuticals having higher photon flux and specific renal handling with less overall radiation dose for these functional measurements. The agents presently available for clinical evaluation of the morphology of the kidney are not entirely satisfactory because of the suboptimal energy of the radionuclide label for use with imaging equipment or relatively high radiation dose or slow rate of excretion with inadequate organ specificity. Many of these agents accumulate only to the extent of 10 to 15~ of the total in the renal cortex of the kidney, and the renal medulla is not visualized adequately by any of the procedures. The clinical investigation of 99mTc-labeled agents besides 99mTe-Fe ascorbic acid that have recently been reported is awaited, although it would seem that the excretion of these occurs at far too slow a rate to offer any real advance in the field. The usefulness of radioisotope procedures for evaluation of renal transplant cases has also been demonstrated with many currently available radiopharmaceuticals, m although the differential diagnosis of rejection, ureteral obstruction, and acute tubular necrosis still remains a major unresolved clinical problem. The use of agents like nSl-labeled fibrinogen in these studies has yet to be more firmly established. Relatively simple techniques using 99mTc pertechnetate have helped a great deal in the understanding and evaluation of urodynamics. Radiopharmaceuticals tagged with the short-lived radionuclides that have ideal physical and imaging characteristics (e.g., 99mTc or ntIn) for kidney evaluation have not been sufficiently explored. It is hoped that future investigations will lead to the development of renal agents with the desired characteristics of providing high target-to-background ratios with short effective halflife and minimal radiation dose.

18

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14°La-DTPA, in: Proceedings of International Congress on Radioisotopes in the Diagnosis of Diseases of the Kidneys and the Urinary Tract, Liege, 1967, ICS 178. Amsterdam, Excerpta Medica, 1967 74. Reba RC, Hosain F, Wagner HN Jr: lndium-! 13m diethylmetriaminipentaacetic acid (DTPA): A new radiopharmaceutical for study of the kidneys. Radiology 90:147, 1968 75. Hosain F, Reba RC, Wagner HN Jr: Visualization of renal structure and function with chelated radionuclides. Radiology 93:1135, 1969 76. Prpic B: Isotope nephrographie mit 6SGa-EDTA in der ratte. Nucl Med (Stuttg) 6:357, 1967 77. Kountz SL, Yeh SH, Wood J, et al" Technetium-99m (V)--Citrate complex for estimation of glomerular filtration rate. Nature (London) 215:1397, 1967 78. Molnar G, Pal 1, Stutzel M, Jaky L: Determination ofglomerular filtration rate with 51Cr, 58Co, 1!4rain, 115rain and 169yb labeled EDTA and DTPA complexes, in: Proceedings of Symposium on Dynamic Studies with Radioisotopes in Medicine. IAEA, Vienna, 1971 79. Pfeifer K J, Rothe R, Bull V, et al: Bestimmung des glomerulum Filtrates mit 169yb-EDTA und des effectiven nieren plasma Stromes mit 131I-hippuran. Fortschr Rontgenstr 117:456, 1972 80. Myers WG, Diener CF: EDTA-SICr gamma ray carrier. J Nucl Med 1:124, 1960 81. Winter CC, Myers WG: Three new testing agents for the radioisotope renogram: DISA-1311; EDTA-51Cr and ttippuran-125I. J Nucl Med 3:273, 1962 82. Garnett ES, Parsons V, Veall N: Measurement of glomerular filtration rate in man using a 51Cr edetie acid complex. Lancet 1:818, 1967 83. Lingardh G: Renal clearance investigations with 51Cr-EDTA and 12Sl-hippuran. Scand J Urol Nephrol 6:63, 1972 84. Favre HR, Wing A J: Simultaneous 51Cr edetic acid, inulin and endogenous creatinine clearance in 20 patients with renal disease. Br Med J 1:84, 1968 85. Brochner-Mortensen J, Giese J, Rossing N: Renal inulin clearance versus total plasma clearance of 51Cr-EDTA. Scand J Clin Lab Invest 23:301, 1969 86. Zum Winkel K, Jahns E, Herzfeld U, Georgi M: Die strahlen Belastung in der nuklear-medizinischen Nierendiagnostik, in: Deutscher Rontgenkongress. Stuttgart, ThiemeVerlag, 1967, p 222

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