Nucf. Med. Eiol. Vol. 16, No. 5. pp. 455-459, 1989 ht. J. Radiat. Appl. hstrum. Part B Printed in Great Britain. All rights reserved

0883-2897/89 $3.00 + 0.00 Copyright Q 1989 Maxwell Pergamon Macmillan plc

99mT~ Ethoxy Methylpropyl Isonitrile:

a Radioactive Tracer for Myocardial

Perfusion Imaging

A. VAN ASWEGEN,‘* A. J. VAN WYK,’ C. P. HERBST,’ A. C. OTTO,’ A. VIVIERS,’ J. KOEKEMOER,3 0. KNOESEN,’ M. G. Lt)TTER,’

P. C. MINNAAR,’ P. H. T. KLEYNHANS’ and P. J. FOURIE’

‘Medical Faculty, University of the Orange Free State, Bloemfontein, 2Atomic Energy Corporation, Pelindaba and ‘Delta G Scientific, Halfway House, South Africa

(Received I December 1988)

The initial experience with a new WmT~ labelled myocardial perfusion agent, ethoxy methylpropyl isonitrile (EMI), is described in the primate as model. Rapid biliary clearance of EMI ensures low radionuclide concentration in the liver and lungs after 60 min. Adequate visualization of the myocardium is therefore possible. Heart-to-lung and heart-to-liver ratios of 1.52 and 2.45 respectively were obtained. EMI is an efficient imaging agent to evaluate myocardial &hernia and infarction,

Introduction

Radionuclide imaging of myocardial perfusion with 20’Tl has over the years become an established method for the diagnosis of myocardial &hernia and infarc- tion (Bodenheimer et al., 1980; Iskandrian et al., 1987). The unfavourable physical properties and cost of “‘TI examinations have however instigated many research programs to find a 99mTc analogue yielding similar biological properties as 20’T1. Recently, several %Tc iabelled isonitrile compounds were introduced i.e. tertiary butyl isonitrile (TBI) (Holman et al., 1984) methoxy-isobutyl isonitrile (MIBI) (Holman et

al., 1986) and carbomethoxy-isopropyl isonitrile (CPI) (Holman et al., 1987). Unfortunately, TBI accumulated largely in the liver and therefore ob- scured evaluation of the inferior wall and apex of the left ventricle (LV). This problem was overcome by MIBI and CPI since these compounds are rapidly excreted through the intestinal pathway. In this paper we introduce another myocardial perfusion agent 2-ethoxy-2-methylpropyl isonitrile which exhibits similar properties to MIBI. Our initial experience in the primate will be reported.

Materials and Methods The isonitrile ether was synthesized following a

procedure as reported elsewhere (van Wyk et al.,

*All correspondence should be addressed to: A. van Aswegen, PhD, Department of Biophysics, UOFS, PCI Box 339, 9300 Bloemfontein, South Africa.

1988). In short, the general procedure is outlined in Fig. 1. In step 1, 2-methylallylamine hydrochloride was formylated with formamide in the presence of triethylamine to give N-2-methyl-2-propenyl form- amide. Then selective hydrolysis in the presence of mercuric acetate followed by reduction with sodium borohydride in ethanol(methano1) gave N-2- ethoxy(methoxy)-2-methylpropyl formamide (step 2). Finally, in step 3, this was dehydrated with phospho- rous oxychloride in the presence of triethylamine to give 2-ethoxy-2-methylpropyl isocyanide (EMI) or 2-methoxy-2-methylpropyl isocyanide (MMI). MM1 is chemically equivalent to MIBI. The structures and quality of all the products were verified using gas-liquid chromatographic, i.r. and NMR tech- niques. Labelling of EMI and MM1 with *Tc was performed by mixing 25 PL of the respective iso- nitriles, 10 mg sodium dithionite and 120 PL ethanol and adding 740 MBq (20 mCi) pmTc]pertechnetate. The solution was autoclaved at 100°C for 20 min. Thereafter 5 mL saline containing 1% Tween-80 was added, the solution filtered with a 0.22 pm millipore filter (yield 5060%) and assayed. Quality assurance of in vitro stability was done on ITLC(SG) with methylethylketone (MEK) and saline to determine 99mT~02 (R,: 0), 99mTc0i (R,: 0.9-1.0) and the *Tc isonitrile complex (MEK: R,: MM1 0.45, EMI 0.8; saline 0).

Five female Chacma baboons (Pnpio ursinus) with a mean mass of 14.8 kg (range 12-17 kg) were studied. After induction of anesthesia with ketamine (10 mg/kg), each baboon was anesthetized for the

NMB Ihs4 455

456 A. VAN ASWEGEN et al.

duration of the study with a constant infusion at 70 mL/h of 0.5% solution of thiopentone in physio- logical saline. EMI was administered in doses ranging from 185 to 259 MBq (5-7 mCi) and the distribution

followed for 60 min. Sixty 1 min images of the thorax in the anterior projection were acquired with the baboon in a supine position using an Ohio Nuclear Sigma 420 scintillation camera equipped with a low energy all purpose collimator and an on-line data processing system (Medical Data Systems A*). Regions of interest were drawn to include the heart, liver and lungs and counts per pixel washout curves generated from each region. Count ratios of heart-to- liver and heart-to-lung were calculated. The same imaging procedure was repeated one week later on each baboon with the administration of MMI.

Single photon emission computed tomography imaging was performed in one case 60 min after EMI administration. Thirty two images of 30s each were obtained through 180” from the right lateral to the left lateral projection using a Siemens ZLC7500 Orbiter camera. Reconstruction was performed by filtered backprojection to obtain slices of 1 pixel (6.4 mm) thickness each parallel to the vertical and horizontal long axis as well as the short axis of the left ventricle (LV).

Results

Figure 2 shows a typical distribution pattern of EM1 up to 60min after administration. At first EMI concentrates in the lungs (Fig. 2a). Twenty minutes later clearance via the biliary system occurred resulting in a high concentration in the gallbladder (Fig. 2b). Lung uptake is still visible. After 40min lung uptake of EM1 has diminished markedly and myocardial concentration is clearly visible (Fig. 2~). Twenty minutes later this situation has improved even further (Fig. 2d). No significant liver uptake is apparent due mostly to biliary excre- tion of the radiopharmaceutical. Corresponding images obtained with MM1 are given in Fig. 3.

The time-activity curves demonstrating washout of EM1 and MM1 from the heart, liver and lungs are shown in Figs 4(a) and (b) respectively. Activity de- creases slower from the heart resulting in an increase in the heart-to-lung ratio with time. The mean values obtained for the heart-to-liver and heart-to-lung ratios for EM1 and MM1 are indicated in Table 1. The radionuclide concentration in the heart increased to 1.52 and 1.11 times that obtained from the liver

$4 STEP1 CH2=C-CH3-NH2

I HCONH2

(‘+H,), N

y3 9 STEP2 CH2=C-CCH2-NH-C-H

I

Hg KH,COO), Na BH4 CH30H/C2H50H

FH3 51 STEP3 CH3- C - CH2-NH-C-H

OR

R= CH3 R= C,H,

I (C2 Ii,), N

POCl3

$4 CH3-C-CH2-N=C

CiR

R: CH, R = C2H5

Fig. 1. Schematic representation of the synthesis of 2- ethoxy-2-methylpropyl isocyanide (EMI) and 2-methoxy-

2-methylpropyl isocyanide (MMI).

for EM1 and MM1 respectively while the heart- to-lung ratios were 2.45 and 3.15 for the two com- pounds respectively. Washout from the liver was thus better with EM1 while MM1 showed superior lung washout.

Reconstructed short axis slices from the apex to the base of the LV with EM1 are shown in Fig. 5. An even distribution of the radionuclide throughout the myocardium is indicated.

Discussion

Myocardial perfusion imaging is an established non-invasive diagnostic examination to evaluate ischemic heart disease and myocardial infarction. The introduction of 99mTc labelled compounds to perform this study is an important development due to its availability, relatively low cost and superior physical properties compared to *“Tl. The higher photon energy of 99mTc causes less scattering and improved image quality. This could eventually lead to better quantitation of myocardial uptake due to the higher

Table I. Mean heart-to-liver and heart-to-lung ratios obtained in 5 studies for EMI and MIBI

Heart-to-liver Heart-to&ma

Time (min) EMI MIBI EMI MIBI

:8 0.77 0.99 f f 0.21 0.42 0.51 0.64 f. + 0.07 0.16 1.71 1.34+0.16 +0.18 2.6610.17 3.05 0.49 * 40 1.37 + 0.58 0.91 + 0.18 2.20 f 0.37 3.09 f 0.90 60 I.52 f 0.66 1.11 f0.19 2.45 + 0.48 3.lSiO.99

Fig. 2. Distribution pattern of EMI at 1, 20, 40 and 6Omin post administration.

Fig. 3. Distribution pattern of MM1 at 1, 20, 40 and 60min post administration.

457

012 60

3oo (b)

225 R

0 -

Time (min)

Fig. 4. Washout curves from heart (circles), liver (solid line) and lungs (crosses) for (a) EM1 and (b) MMI.

Fig. 5. Tomographic short axis slices obtained with EMI.

458

Myocardial perfusion imaging with 99mTc EM1 459

count rate obtained. Furthermore, 99mTc isonitriles

permit functional and perfusion imaging to be per-

formed simultaneously. Functional imaging can either be performed during the first pass of the radionuclide through the LV after administration (Balino et al., 1987) or during gated acquisition of perfusion images and identification of the LV cavity

on each image to calculate ejection fraction (Chesler

et al., 1987). Although no specific synthesis of MM1 was found

in the literature, our own scheme of synthesis for the

isonitriles proved to be very reproducible. The dehy- dration step using phosphorous oxychloride was also

more convenient and less hazardous than known literature methods using phosgene gas (Ugi er al., 1965). In vitro stability of the 99mTc isonitriles was checked using the quality control methods mentioned and showed no dissociation for a period of up to 3 h after labelling. HPLC analysis on the labelled com- pounds generally gave single peaks in less than 10 min. In uioo stability was monitored in a relative way during the different studies. Since no significant residual uptake was noted in the liver and spleen (potentially due to colloidal 99mT~02) and also no uptake in the stomach and thyroid (potentially due to 99mTc0;) it was concluded that no in vioo degrada- tion of the 99mTc isonitriles had occurred over the study period.

The liver uptake of EMI after 60 min was low and resulted in an unobscured evaluation of the inferior wall and apex of the LV. In this respect EM1 was superior to MMI. MM1 however, showed an im- proved heart-to-lung ratio compared to EMI. The residual lung concentration of EM1 was insignificant and did not pose any disadvantage when viewing myocardial uptake.

A further advantage of the isonitrile compound is that it does not redistribute into zones of transient ischemia for a considerable period of time after exercise (Zaret and Wackers, 1987). MM1 especially has a prolonged washout from the myocardium which necessitates the rest study to be performed on a separate day. The washout of CPI is however much more rapid and the rest study can usually be per- formed 4 h post exercise (Holman er al., 1987). The washout of EM1 has only been evaluated for 1 h in the present study and the prolonged distribution pattern is currently under investigation,

In this study we report the use of y9mTc labelled EM1 as a myocardial perfusion scanning agent. Ex- cellent images were obtained 60 min post administra- tion in the primate model. These results are promising for the use of EM1 for myocardial perfusion imaging in humans.

Acknowledgemw--This work was performed as part of a research grant from the South African Medical Research Council.

References Balino, N. P.; Sporn. V.; Kuschner. E.; Masoli. D.:

Castro, R.: Troilo. A.; Mitta, A.; Camin. L. Simultaneous test of ventricular function and myocardial perfusion with 99mTc RP-30. J. Nucl. Med. 28:662; 1987.

Bodenheimer, M. M.; Banka, V. S.; Helfant. R. H. Nuclear Cardiology. II. The role of myocardial perfusion imaging using Thallium-201 in diagnosis of coronary heart disease. Am. J. Cardiol. 45:674-684; 1980.

Chesler, D. A.: Boucher. C. A.; Strauss, H. W. Automated computer derived left ventricular ejection fraction from gated myocardial (RP30) images. J. Nucl. Med. 28:662; 1987.

Holman. B. L.: Jones, A. G.; Lister-James, J.; Davison, A.; Abrams, M. J.; Kirshenbaum, J. M.: Tumeh, S. S.; English, R. J. A new 99mTc-labelled myocardial imaging agent, Hexakis(t-Butylisonitrile)-Technetium( l)pmTc TBI]: Initial experience in the human. .I. Nucl. Med. 25:135&1355; 1984.

Holman, B. L.; Jones, A. G.; Davison, A.; Rigo, P.; Sporn, V.; Vosberg, H.; Moretti, J. Comparison of three %“Tc isonitriles for detection of ischemic heart disease in humans. J. Nucl. Med. 27:878; 1986.

Holman, B. L.; Sporn. V.; Jones, A. G.; Sia, S. T. 9.; Balino, N. P.; Davison, A.; Lister-James, J.; Kronauge, J. F.; Mitta, A. E. A.; Camin, L. L.; Campbell, S.; Williams, S. J.; Carpenter, A. T. Myocardial imaging with Tech- netium-99m CPI: Initial experience in the human. J. NW/. Med. 28:13 -18; 1987.

Iskandrian, A. S.; Heo, J.; Askenase, A.; Segal, 9. L.; Helfant, R. H. Thallium imaging with single photon emission computed tomography. Am. Heurt J. 114:852-865; 1987.

Ugi, 1.; Fetzer, U.; Eholzer, U.; Knupfer, H.: Offermann. K. New methods in preparative organic chemistry IV: Isonitrile syntheses. Anger. Chem. Int. Ed. 4:472-484; 1965.

van Wyk, A. J.; van Aswegen, A.; Knoesen. 0.; Fourie, P. J.i Koekemoer, J.; He&t, C. P.; Otto, A. C.; Latter. M. G.: Minnaar. P. C. Svnthesis and Y9mTc labelline of MM1 (MIBI) and its ethyl analogue EMI. Appl. Raiiar. Isot. Submitted.

Zaret, 9. L.: Wackers, F. J. Radionuclide methods for evaluating the results of thrombolytic therapy. Cirtulution 76 (Suppl. II); 11-8-11-17; 1987.