Carbon-11: Where familiar chemistry still holds new challenges

Organic synthesis for biomedical applications with carbon-11 is made feasible by the high activities which can be cyclotron produced compared to the q...
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Carbon-11: Where Familiar Chemistry Still Holds New Challenges Anthony L. Feliul Depaltment of Neurology, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021

I t is onlv natural t o assume that anv routine discussions of organic reactions involve carbon-li. Carbon-12, after all, comorises 98.89% of the naturallv occurrine carbon. In total. eight isotopes of carbon are knbwn (see Table 1). ~ e s i d e s carbon-12, two others occur naturally: the stable carbon-13 and the long-lived beta-emitter carbon-14. Of five artificial isotopes, carbon-9, carbon-10, carbon-15, and carbon-16 have extremely short half-lives-on the order of secondsand the last, carbon-11, a leisurely 20 min. As substrates for organic synthesis, only in recent times has work with enriched isotopes of carbon been practical, as carbon-13- and carbon-1Clabeled Drecursors became readily available, off-the-shelf items. ~ i e r still e remains carbon11though-its half-life far too short to be a stock item, but nevertheless, tantalizingly long. I say tantalizing because, as this paper describes, imaginative chemists have been able to overcome many challenges imposed by this short-lived radioisotope and are pushing the frontiers of the traditional synthetic art to label a host of biologically interesting compounds. The growing interest in carbon-11 chemistry parallels developments in several other disciplines, including solid-state physics, biomathematics, and computer science, which together are gradually transforming positron emission tomography (PET) from a purely research technique into one of mainstream clinical practice. By means of radiotracers, P E T orovides a noninvasive method for selectivelv observing ~hy~iological processes and, as such, holds the kky to understandins fundamental biochemistry in normal and diseased tissue (7). Exotic as this nuclide may first sound, carbon-11 bas, in fact, been used routinely for some years now a t research P E T centers around the world. And with new synthetic methods under development, carbon-llE'ET promises to play an ever-increasing role in diagnostic nuclear medicine as we approach the next century. Properlles ofCarbon-11 Among all known radioisotopes, perhaps carbon-11 is uniouelv suited to assume a maior role in biomedical research &d clinical diagnosis with^^^. First, carbon is the basic element of all life. I t is therefore possihle (at least in principle) to tag any desired biologically active molecule withcarhon-11 and introduce this radiotracer into the living system without perturbing normal pharmacologic function. Second. the radiation emissions are suitable for external detection. Although positrons (positive electrons, positively chareed beta narticles) do not ~ e n e t r a t elivine tissue more than-a couple bf milliieters, thk pair of high-energy gamma ravs resultine from oositron-electron annihilation are bodv-and c& be detected easily with an external scintillation detector svstem. Carbon-14, tritium, and other radionuclides that decay by emission ofnegatively charged beta articles are unsuitable for diaenostic medical application because, among other reasons, their radiation-Ls not body-penetrating. 364

Journal of Chemical Education

Table 1. Iwtopes 01 Carbon (2). Isotope carbon4 carbon-10 carbon-11 carbon-12 carbon-13 carbon-14

Abundance

98.89% 1.11% P P ~

carbon-15

HalCllfe

Decay Mods

0.1265s 19.28 20.38 mln stable stable 5730 y

Btp2m

B-

2.449 s

8-

B+ B+

. . panlcle (parltron) and a neutrino:

In n s m m l c h nuclides, &cay ocovraas s neutronarnvorUto a pram wlm release of a negstlve beta particle sM an antineutrino: Carbon-14

IBP 8")

+

Nltrmnen-14 I7p 7")

+F +?

Third, the short half-life effectively limits the radiation dose to the patient and permits repeat studies within relatively short intervals. Fourth, the short half-life endows the radioisotope with a high intrinsic specific activity. The term specific activity is formally defined as the ratio of radioactive atoms t o the total number of atoms of agiven element (2).In practice, however, i t is often convenient to refer to specific activity as the quantity radioactivity per mole of sample, this being proportional to the atomic composition and inversely proportional to half-life2. High specific activity is a generally desirable characteristic for radiotracers used in biomedical aoolications because the quantity of radioactivity needed tiobtain satisfactorv countine statistics will be associated with a correspondingly s m a l l k a s s of tracer. Given that individual com~oundsin livine svstems are Dresent in minuscule concenGations, it is essential to limit the mass of radiotracer administered to avoid perturbine- the hiolosical un- orocess . der study. And fifth, large amounts (radioactivity) of carbon-11 are easily generated in the form of chemically useful precursors.

Present address: lnstitut fur Nuklearchemie, Kernforschungsanlage Jcllch GmbH, Postfach 1913, D-5170 Julich. Federal Republic of Germany. Intrinsic speclfic actlvifyrefers to the activity per unit mass of a radionuclide when it is isotooicallv . , .oure. This auantlh, is solelv a function of half-life(10). It is interesting to compare'thi lntrin& spec.ficactivities for carbon-11 (9.23 X lo9 Cllmol) and caroon-14 (62.4 Ci/mol). Tnus 20 mCi pure caroon-t 1 corresponds to on,y 2.17 X mo but 20 mCi carbon-14 is 3.2 X lo-' mol.

.

Salient Points In Carbon-11 Chemistry Carbon-11 presents the chemist with several unusual challenees c o m ~ a r e dto carbon-12 (or even carbon-13 and carbon-14 f i r that matter). While the 20-min half-life is the obvious constraint on the chemist's imagination, specific activity, stoichiometry, purification methods, and radiation ~rotectionare additional considerations. Organic synthesis for biomedical applications with carbon-11 is made feasible by the high activities which can be cyclotron produced compared to the quantity required for a patient study. Starting with 1 Ci, forexample,asynthesis/purification procedure that requires 1 h (three half-lives) and that has only a 25% chemical yield would still afford 31 mCi of final product--sufficient for a patient study. Carbon-I2 is ubiquitous in the environment. It is not only present in the atmosphere as carbon dioxide, but in a myriad of other forms. Although the connection between carbon-12 contamination during a radiosynthesis starting with ["Clcarbon dioxide is apparent, radiolytic decomposition processes also release minute amounts of carbon-12 from complex organic compounds too, and such carbon dilutes the nuclidic purity of the final product. Apparently "trivial" sources of carbon-from target gas supply to reagents-are, in fact, troublesome sources of carbon-12 that reduce the s ~ e c i f i cactivitv (i.e.. nuclidic ~ u r i t v )of the I"C1-labeled product. consiber &at 1 Ci o i purdcarbon-il represents onlv 1.08 X lo-" mol but that 1r L carbon dioxide a t stand& temperature and pressure is 2.2 X lo-'' mol! Undesired environmental contaminant&are so difficult to exclude that even under the most rigorous conditions, no more than 111,000 or 1/10,000 molecules in a given [W-labeled compound will carry a carbon-11 atom; the remainder will have carbon-12. The foreeoine discussion of s~ecificactivitv raises a relat" ed point concerning reaction stoichiometry. Since 1 Ci of I11Clcarbon dioxide with a s~ecificactivitv of 111,000 reuresenis a mere 10.8 nmol of &ting materid, i t is unavoidible that reaeents will be resent in large excess and that reactions w'il be conduc.ted under h k h dilution conditions. Techniques to scale down chemical reactions involve careful planning and experimentation, as standard reaction conditions often require substantial modification to succeed at tracer concentrations. . - ~ ~ ~ - ~ Similarly, isolation and purification methods must be rapid and extremelv efficient because the mass of labeled oroduct is generally small compared to reaction byproducts and unconsumed reagents. For most syntheses, preliminary isolation of the product is accomplished with a solid phase extraction technique or flash chromatography, and then preparative high-pressure liquid chromatography (HPLC) is used for final ~urification. Finally, theie is an issue of radiation protection. The 511keV annihilation radiation is of benefit for imaging purposes but poses a hazard to the chemists. ~ccordingly,~skntheses are performed in radiation-tight hot cells lined with 5 cm lead and fitted with 10-em-thick lead glass windows. While experiments are sometimes done using tongs or masterslave manipulators, for speed and convenience the chemist will most frequently custom design a remote- or computercontrolled a~Daratusfor the routinelv used radiotracers. ~~~

u

~

~

Generating Bask Carbon-1 1 Precursors

Transportation and storage of carbon-11 is effectively precluded by its 20-min half-life, so chemists generate carbon-11 as needed with a particle accelerator, usually a cyclotron (3). Carbon-11 is most commonly produced by proton bombardment of nitrogen gas. The terminology for expressing this nuclear reaction takes the form '"(p,

u)"C

Table 2.

Representative ["C]Radlopharmaceullcals and t b l r Appllcatlon In PET

Radlohacer

Symhetic Scheme

[I-"Clpalmltic acid

["C]carboxylation of Orignard reagent o~-[t-"C]~henylalmIn%["C]carboxylation of lithi~l~onihile o~-["C]rvmsiw Bucherer-Strecker rsaclion wim [ ' ClWN [2-"C]s.s-dimethyIoxa- acylatm of ["Clphos zolidin-2.4-dlone gene [I-"C]2deoxygluwse Kllianl-Fischer-type chernistv with ["CIHCN

intended Application myocardial fany acid

utilization

protein symh/amino acid Irans~OR prole n synlnlamino acid han~pon csrsoral pA

measurement cerebral giumse utilization

+

auylonltrile ["CIHCN malignant t u r n marker wnh B2He reduction dopamine D2-recepN["C]melbylspiperone ["C]methyllodide Ntor ligsnd alkylation [1tCt7u-methyl]mst~ ["C]rnethyllnhlurn + D e steroid receptor ligand tosterone-3-ketal sterone [I-"C]putresdne

therebv,showine that a nitroeen-14 atom is caused to react with an inciden;proton tog&, after ejectingan alpha (helium-4) oarticle. a carbon-11 atom. The "N(D. aJ"C reaction is qui& efficient, and typically 1or 2 Ci of carbon-11 can be eenerated during a 30-min bombardment with a compact i;iomedical cyclo&on. The definition for a curie of radioactivitv.3 3.7 X 10lodisinteerations per second, is too abstract to c&ey its true magGitude, b& this is indeed a large quantity to handle. From an unshielded 1-Ci source, for example, even at arm's length a person would sustain a whole-body radiation dose equivalent to a chest X-ray in just seconds. The curie of carbon-11 generated by the cyclotron hombardment compares with a 20-30-mCi patient dose of the final labeled radio~harmaceutical. The nuclear reaction is only the first of several steps that make carbon-11 available t o the svnthetic chemist. Initiallv. the carbon-11 atomsproduced by ihe nuclear reaction have-a recoil enerev of several MeV, and this kinetic eneray is dissipated through collisional deactivation and ''&t atom" chemical reactions. The recoil energy can be exploited if the cyclotron target chamber is filled, not with pure nitrogen, but with nitrogen containing part-per-million levels of either oxygen or hydrogen. During the cyclotron bombardment these gases will combine with the recoil carbon-11 monoxide atoms to produce ["Clcarbon di~xide/[~lC]carbon or ["Clmethane, respectively. After irradiation, the target eases are ~rocessedthroueh catalvtic furnaces and then coniucted td the hot chemistry laboratory. On line passage of the PVlcarbon dioxide1l~'Clcarbon monoxide mixture tbroigh 'copper oxide a t 8'00 ;C converts i t completely to ["Clcarbon dioxide, while the use of zinc turnings a t 400 'C gives ["Clcarbon monoxide. [Wlmethane is routinely converted into ["Clbydrogen cyanide by addition of ammonia and passage through platinum wool a t 1000 OC. These three basic precursors, ["Clcarbon dioxide, ["Clcarbon monoxide, and ["Clhydrogen cyanide, form the foundation for all carbon-11 chemistry. ~~~~

~

~

~~~~

Organic Synthesis wlth Carbon-11 Desoite the foreeoinechallenees, the chemical literatureis of carbon:ll-labeled radiopharmaquite kch with ex&& ceuticals-more than 400 have been avntbesized to date. Table 2 presents a sampling of these radiotracers and their The deflnkion for 1 Ci of radioactivity was originally taken from me decay rate for ig of radlum, but it Is now applied to any radioactive sample wlth the same number of disintegrations per second. Volume 67 Number 5 May 1990

365

-

R

'CH3Li

C OH i'CH s

2

Figure 1. Schematic overview of mmmon synthetic routes, starting with ["C]carbon dioxide.

biomedical applications in PET. Figures 1 and 2 present schematic overviews of the most common synthetic routes starting with either ["Clcarbon dioxide or [llC]hydrogen cyanide. T o illustrate how traditional chemical reactions can he a d a ~ t e dto tracer concentrations and optimized for speed, severai examples will be presented. The reader is encou;aged to exdore the literature independently for additional information (1,4,5). [llC]Carboxylation of a Grignard or organolithium reagent offers a simple route t o [llC]carboxylic acids. Similarly, aliphatic and aromatic ["C]DL-a-amino acids are readily synthesized by [llC]carboxylation of a-lithioisonitriles. The reactions are rapid and proceed in high yield. The preparation of ["C]palmitic acid (including workup and formulation for patient use) from [llC]carbon dioxide and pentadecylmagnesium chloride, for example, can be performed in 15 min with approximately 75%radiochemical yield, decay corrected. Rapid synthesis and purification of this tracer are made possible via a captive phase technique (61,in which gaseous [llC]carbon dioxide passes through Grignard reagent preabsorbed with minimal tetrahydrofuran solvent on microporous polypropylene powder. The alkylation of amines, alcohols, and mercaptans with [llC]methyl iodide is another commonly employed labeling method. [llC]methyl iodide is available from [llC]carbon dioxide in a two-step, one-pot procedure requiring 5-10 min and proceeding in about 90% yield (7):

Because alkylations are operationally easy to perform and methyl groups are so common in biologically active compounds, researchers have expended considerable effort to optimize the [llC]methyl iodide synthesis. For instance, the specific activitv of I1lClmethvl iodide is known to he denendent on the qu⁢of the lithium aluminum hydride, w'hich avidly absorbs carbon dioxide if handled in the open atmosphe;e, and on complete evaporation of the tetrahydrofuran after the reduction step. Typically, [llC]carbon dioxide is passed into a solution containing lithium aluminum hydride (1 mg) in dry T H F (0.5 mL). Next, a heating bath is raised to evaporate the solvent, leaving a solid residue of [llC]methoxide. Addition of concentrated hydriodic acid (0.5 mL) to the residue and continued heatine under a nitroeen stream effects simultaneour hydrol&is and iodination t o give ["Clmethyl iodide. The latter is conducted into a solution of

-

366

Journal of Chemical Education

Figure 2. Schematic overview of common synthetic routes startlng wlth ["C]hydrogen cyanide.

~~

~~~~~~~~

the substrate (2-5 mg, often in DMF or acetonitrile) and base (tetraalkylammonium hydroxide, sodium hydroxide, or potassium carbonate) for the methylation. After a suitable reaction period (5-20 rnin), the reaction mixture would be iniected directlv into a ~ r e ~ a r a t i HPLC ve for isolation of the [li~)labeled priduct. dverall, ["C]methylations requm 2045 min from end of cvclotron bombardment iEOBI LO final formulation for patient use. Incidentally, an alternative [llC]methylation sequence, starting with [llC]formaldehyde, is available as outlined below:

The synthesis of an alkylamine from [IIC]cyanide is ilhw trated by the two-step synthesis of [llC]putrescine (8):

First, [llC]hydrogen cyanide is trapped in aqueous potassium hydroxide (0.2 mL, I%), a 50%solution of acrylonitrile in T H F (1mL) is then added, and the mixture is heated a t 65 "C for 5 min to give [llC]succinonitrile via conjugate addition. Next, the reaction mixture is evaporated to dryness and exposed to horane-dimethylsulfide in T H F (2.5 mL) a t reflux (10 min). Residual borane is quenched with methanolic HCI and the [llC]putrescine is isolated by preparative ionexchange HPLC. [llC]Putrescine is obtained in approximately 20% yield, decay-corrected, in 50 min from end of cvclotron bombardment (EOB). "The synthesis of ["~~,5-dimeth~loxazolidine-2,4-dione (DMO) is interesting because it demonstrates the feasibility of work with tracer concentrations of an intermediate having high chemical reactivity, ["Clphosgene:

[llC]Phosgene has been generated on-line from [llC]carbon monoxide both catalytically, by reaction with platinum(1V) chloride at 380 "C, and photochemically, with molecular chlorine and ultraviolet light. T o synthesize [llC]DMO (9), [llC]phosgene is bubbled into a suspension of 2-hydroxy-2methylpropionamide and sodium hydroxide in acetonitrile a t -5 OC over 20 min. After being warmed to room temperature, the reaction mixture is evaporated to dryness, dis-

solved in p H 8 buffer, and filtered through a plug of reversephase chromatographic packing material. A t p H 8, ["CIDMO, being ionized, passes through the column unretained, while the impurities, being un-ionized, remain behind. Overall, this route affords a 4040% radiochemical yield of [I'CIDMO in a synthesis time of 40 min. Conclusions Although many compounds have been labeled with carbon-11, this field is obviously in its infancy compared to traditional organic synthesis. There is much room for immovements and new ideas. For example, radiochemists have bnly begun to explore solid supports to handle certain reagents, a strategy that could simplify isolation and purification problems. Many versatile reagents for carbon-carbon bond formation, organohoranes and transition organometallics, mentioning just two, remain to be exploited. Further research in hot atom chemistry may make more elaborate building blocks availahle on-line from cyclotron targets. And further development of automated hardware will enable chemists to spend more time on basic research and less on routine production chores. Synthetic organic chemistry with carbon-11 presents both risks and rewards. Yes, a complex multistep sequence can be done in 15-45 min, hut the radiochemist may spend as many weeks developing it. Lurking in the background are potential oitfalls: soecific activitv. radiation orotection. radiopharmaceutich formulatioifor human ise-to n&e just three. T o work with carbon-11, the chemists need access to a cyclotron, a hot lab, and an array of other capitalequipment. Radiochemistrv with short-lived positron emitters (and carbon-11 is just one) is surely not fbr the light hearted.

.

~~

What such chemistry does offer is scientific diversity and stimulation-thisat a time when the trend is toward specialization. Although much of this discussion has related to svnthetic asoects of carbon-11. all such ~ .r o"i e c t sare closelv allied with-and dependent on--expertise in nuclear chemistry (cyclotrons, targetry, hot-atom chemistry), engineering (remote synthesizers, automated data acquisition), applications programming (instrumentation control, data analysis), pharmacology (in vivo radiotracer evaluation, metabolite analysis, dosimetry), and clinical medicine. And the chemist, though not expert in all these areas, will, as part of a larger research team, be an active contributor. Working on a project from its first conception, through chemistry, through biological evaluation, and on to clinical application-that makes this radiochemistry uniquely worthwhile. Llterature Clted 1. For further discussion andieadingreferences. see:Feliu.A. L.J. Chem.Educ. 1988.65,

655-660. 2. F,i.dlender.G.;Kcnnedy. J. W.;Msciss,E. S.;Mi1ler.J. M.NueleorondRodioehsmrrtry: Wiley:

New York. 1981.

3. Biratfari, C..Bonardi, M., Ferrsd, A., Milanesi, L.. Silsri. M J. Med Eng Technol. 19.97. I 1 (JulylAug) 166176. 4. Wolf, A. P.; Reduanly, C. S. Inl. J. Appi. Rodiof.Isof. 1977.28.2948. 5 . Stkkiin, G. "Spezielle Synthoseverfahren mit kurrlebigen Radionukliden und Quaiitatakontrollc"InHandbuchdrrm~dirinirrhenRodiiIo~t~, BandXVIIB. Diethelm, L.: Heuck. F.: Olsson, 0.; Sfmad. F.: Viefen. H.; Zuppinger, A , Eds.: Springer: Berlin, 1988; Chap,- 1.2. 6. Jewett,D.M.; Eh1enksufer.R. E.:Ram.S.Int.J.Appl.R~diol.lsof.1985.36.672-674. 7. Cmuzei, C.; Lsngatrom. B.: Pike. V. W.; Coenen. H.H. Appl. Radial. IsoL 1987, 38.

"".

f"lLiiOl

W.;Fowler,J.S.:Wolf,A.P.:Amott,C.D.:Bradie. J.D.:Volkow,N.J. NuclMed. 1985,26,11861189. Dik3ic.M. Inf. J. Appl.Rodiel.Isof. 1984,35,103&1037.

8. McPherson.D. 9.

10. Friediandc? and Kennedy. p7.

Michael Faraday: Chemist and Popular Lecturer The 200th anniversary of the birth of Michael Faraday will be marked by a symposium titled "Michael Faraday: Chemist and Popular Lecturer". Co-sponsored by the Division of Chemical Education and the Division of History of Chemistry of the American Chemical Society, the symposium will take place on April 15116,1991at the Society's Spring Meeting in Atlanta, Georgia. Faraday served his scientific apprenticeship under a chemist, grew to scientific maturity ss one of the first professionsl chemists, and held the Fullerian Professorshipof Chemistry at the Royal Institution from 1833 until his retirement in 1861. Even so it is for his achievements in physics rather than in chemistry that he is chiefly remembered. Faraday himself would have deplored such nice distinctions, much preferring the term "natural philosopher", but in this age of specialization we plan, as our title implies, to leave the celebration of Faraday's seminal contributions to physics largely to others. Much still remains: the isolation of benzene and isoprene, the sulfonation of naphthalene, the preparation of polyhalogenated hydrocarbons, the liquefaction of gases, the demonstration of the paramagnetism of gaseous oxygen, the improvement of optical glasses and corrosion-resistant steels, and, above all, in his statement of the law(s) of electrolysis, the forging of the first quantitative link between chemistry and electricity. Furthermore, to increase the popularity (and the income) of the Royal Institution, Faraday instituted the Friday Evening Discourses and the famous Christmas Lectures addressed to a "juvenile" audience. Both series continue to the present day and have long served as exemplars for the popularization of science. I.. Pearre Williams, the doyen of Faraday srhdars, hasagrerd rolead uffrheaymposium, which will be made upofhoth invited and contributed paper*. I'ublicarion is planned. Persons intercrrd in parliripntin~are invited t o contact Derek A. Dsvenport. Department of Chemirlry, Purduc IInrversiry, \Vest l.afnyette, IU 479Uq, (3171491-5465.

Volume 67

Number 5

May 1990

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