Abstract:
The present invention relates to the field of site directed therapy. More specifically the invention relates to site directed radio therapy, and provides a method for production of radioconjugates and an apparatus for radioimmunotherapy. The method, conjugates and apparatus can be practicalized without the need for radioactive shielding and/or airtight facilities. Without these restrictions the invention provides a simple and efficient means of therapy at the bed-side of the patient.

Description:
[0001]    The present application is a continuation-in-part of co-pending U.S. patent application Ser. No. 08/440,857, filed May 15, 1995, which is a division of U.S. patent application Ser. No. 08/097,471, filed Jul. 27, 1993, and now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 07/657,580, filed Feb. 19, 1991 and now U.S. Pat. No. 5,246,691. The entire disclosures of all of the above-mentioned patent applications are incorporated by reference in the present specification as though set forth herein in full. 
     
    
       [0002]    The present invention relates to the field of site directed therapy. Nowadays there are a number of methods of site directed therapy which have been suggested to eliminate unwanted cells or other biological matter, e.g., vascular occlusions, or infectious organisms from the body of a mammalian subject. 
         [0003]    There are many fields of therapy in which such methods may be applied. The most important ones include, without limitation, immune diseases (either auto immune diseases or acquired immune diseases), cancer and viral or microbial infections. 
         [0004]    Site directed therapy is a method whereby a cytotoxic compound is delivered to the immediate vicinity of the target cell or infectious organism. This is usually done by coupling a targeting moiety to the cytotoxic compound. 
         [0005]    This targeting moiety recognizes a structure in, on, or near the target. Known targeting moieties include, but are not limited to, antibodies, more specifically monoclonal antibodies and more preferably human monoclonal antibodies, nucleic acids, receptor directed ligands and the like. 
         [0006]    Cytotoxic compounds can be for instance drugs, such as adriamycin, toxins such as ricin A and radioisotopes. 
         [0007]    Radioisotopes can be used not only for therapy, but also to identify the site or sites of the target (imaging). This invention provides methods of therapy and imaging using a conjugate of a targeting moiety and at least one radioisotope. 
         [0008]    Therapy with targeting moieties is widely known. Targeting can be accomplished by aiming the targeting moiety directly to the wanted site, but it may also be directed to another targeting moiety which is directed to the wanted site (so called pretargeting). Pretargeting offers an advantage over direct targeting when the specificity of the targeting moieties is not sufficient. By using a first localizing moiety followed by a second one coupled to a cytotoxic compound, the amount of cytotoxic compound delivered to non-target sites can be lowered significantly. 
         [0009]    Known targeting moieties, such as antibodies, often cannot be provided with a large amount of cytotoxic compounds without hampering their targeting specificity. 
         [0010]    Therefore it has often been suggested to use a carrier molecule, such as HSA or a nucleic acid, or a polymer, which can be loaded with a high number of cytotoxic compounds and coupled to a targeting moiety. 
         [0011]    All of the above-mentioned variations on the theme of site directed therapy and/or imaging can be used more advantageously with the present invention. 
         [0012]    A well established problem in the field of imaging and site directed radiotherapy is to find a suitable radioisotope. Apart from the amount of energy that is released upon their decay, which should be sufficient to be measurable outside the subject in the case of imaging and sufficiently lethal to the target in the case of therapy, there is also a problem in finding an isotope with a suitable half-life. 
         [0013]    An isotope with a long half life cannot be chosen because of the biological half life of the targeting moiety, which means that most of the isotopes will decay after disintegration of the conjugate. This decay after the disintegration of the conjugate will lead to cytotoxicity to other cells or tissues than the target. 
         [0014]    Furthermore, all conjugates which do not localize will be secreted from the body and present a radio active waste problem. 
         [0015]    It is also not practical to choose a radioisotope with too short a half life, because of packing and shipping delays and because the institution carrying out the therapy must be equipped to make the conjugate, transport it to the patient and administer it in a very short interval of time, otherwise most of the radioisotope will have decayed before entering the body, let alone localization at the target site. 
         [0016]    The isotopes used for imaging usually are gamma emitting isotopes, for therapy auger electron emitting α-β-emitting, or α-emitting isotopes may be used. 
         [0017]    Most preferred for the present invention are α-emitting isotopes. 
         [0018]    The short-range cell-killing effect of α-particles is enormous: a 1 mm diameter tumor, comprising maybe 600,000 cancer cells needs about 6 α-particles of 6 MeV per cell to deliver a dose of 600 red, causing a 99.9% cell-kill ratio, and that specifically because of the stochastic nature of the hit- and kill-mechanism. 
         [0019]    However, due to the same stochastic nature, a 10 times lower a-radiator dose will enhance the cell survival ratio with a factor 500: more than 50% of the cells (or non-tumor cells in similar morphology for that matter) would survive a 60 red α-radiation dose, equivalent to 0.6 α-particles per cell. 
         [0020]    This characteristic would make an effective α-radioimmunotherapy within reach, provided that a “quality factor” for the isotope-antibody conjugate of 10 or better can be obtained. It is the purpose of the present invention to contribute towards this goal in a most essential manner. 
         [0021]    The quality factor is a ratio between localized antibody at the target site, divided by the antibody “sticking” to other tissue. 
         [0022]    The notion of using α-particle emitting radioisotopes as agents for the killing of tumor cells was already mentioned in the literature during the mid-fifties. Since then other potential candidate isotopes were and are being proposed, of which a good summary is given by Fisher (1) and by Wilbur (2) which brings the list to (with their half-lives between parentheses):  223 Ra (11.4 d),  225 Ac (10 d),  224 Ra (3.6 d),  225 Fm (20 h),  211 At (7.2 h),  212 Bi (60 m), and  213 Bi (47 m) 
         [0023]    Although important publications appear regularly in the literature regarding microdosimetry, antibody isotope coupling techniques, pre-clinical in vitro and in vivo experiments, no clearly defined, larger scale clinical experiments are being done until now, for a variety of reasons: 
         [0024]    a. very few human monoclonal antibodies with proven sufficient quality are available yet, 
         [0025]    b. very limited biological safety data are available for antibody coupling agent (the latter for the binding of the radioisotope) combinations, 
         [0026]    c. some isotopes may not become available for large scale application at acceptable cost prices ( 225 Fm), 
         [0027]    d. isotopes may be too difficult and therefore too expensive to obtain because of the necessary procurement process ( 211 At from  209 Bi by a (α, 2n) reaction in a cyclotron and subsequent isolation plus purification), 
         [0028]    e. other isotopes do have a Rn-isotope as first daughter in their decay sequence, allowing redistribution of daughter nuclei before decay ( 224 Ra,  223 Ra), and also necessitating gas-tight reaction conditions, 
         [0029]    f. some isotopes may have a relatively long living daughter isotope somewhere in their decay sequence ( 224 Ra,  223 Ra,  225 Ac) also with the chance that daughters thereof may redistribute before decay, 
         [0030]    g. the radioactive halflife of some isotopes is so long that most of the activity leaves the patient undecayed, resulting in a waste problem ( 223 Ra,  224 Ra  225 Ac) or 
         [0031]    h. the halflife of the isotope is so short that most of the isotope decays before reaching its ultimate target ( 212 Bi,  213 Bi), 
         [0032]    i. sufficient precursor material may not be available to extract at one time the necessary amount of isotope for a single patient treatment ( 212 Bi,  213 Bi), and, 
         [0033]    j. isotopes producing hard gamma rays in their isotope decay need shielding facilities to prevent radiation hazards to technicians and nursing personnel. 
         [0034]    One or more of the arguments listed above will make it very difficult, if not impossible for some of the isotopes to ever be used on a large scale for α-radio-immunotherapy, and that in particular if one or more of the others can be used on acceptable technical, logistical and financial conditions. 
         [0035]    The closest prior art to the present invention (the French patent application FR-A-2 527 928) discloses a method to produce conjugates of an antibody and  212 Bi. However, these conjugates still suffer from the drawbacks mentioned above under e, h, i and j. 
       SUMMARY OF THE INVENTION 
       [0036]    The present invention provides a therapeutic radioconjugate of a targeting moiety bound, directly or indirectly, to at least one relatively short lived α-particle emitting radioisotope, the targeting moiety being effective to deliver the conjugate to a pre-determined pathological site in a mammal. The pathological site comprises a target moiety, which together with the targeting moiety, constitutes a specific binding pair. 
         [0037]    The targeting moiety is preferably selected from the group consisting of an antibody, or antibody fragment which bind specifically to a cell-surface antigen; or a ligand or ligand fragment which bind specifically to a cell surface receptor. 
         [0038]    The pathological site may comprise diseased cells, e.g., tumor cells, cells at an inappropriate site, e.g., neutrophils at site of inflammation, platelets at sites of injury, or it may be an extracellular structure, such as fibrin or thrombin within a coronary vessel at a site of infarct, cholesterol at a site of atherosclerotic plaque, calcification in diseased vessels, amyloid plaque at disease sites in the brain, or the like. 
         [0039]    Also according to this invention, a method is provided for preparing the above-identified therapeutic radioconjugate. The method involves loading onto a binding medium a relatively long lived radioisotope, the decay sequence of said relatively long lived radioisotope yielding daughter isotopes that emit predominantly α- and/or β-particles without harmful gamma radiation, eluting said α-particle emitting radioisotope from said binding medium and coupling said α-particle emitting radioisotope, substantially free of said relatively long lived radioisotope, to said targeting moiety. 
         [0040]    Relatively long lived means that the radioisotopes have a decay time in the order of several days, which enables sufficient time for packing and shipping. Relatively short lived in this context means that the radioisotopes have a decay time in the order of minutes or hours. 
         [0041]    The reference to a decay sequence in which predominantly α and/or β-rays are emitted is meant to signify a decay sequence which does not cause danger for radiation hazards caused by gamma rays to people working with the compounds without the burden of applying protective shielding. 
         [0042]    Preferably, the relatively long lived isotope is  225 Ac and the relatively short lived α-particle emitting radioisotope is  213 Bi. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]      FIG. 1  depicts an apparatus which may be used in practicing this invention. 
           [0044]    An eluent containing a targeting moiety passes from a vessel ( 1 ) through an ion exchange column ( 3 ), containing a bound parent radiometal, wherein a daughter radioisotope binds to the targeting moiety. The eluent containing the conjugate is mixed with infusion liquid from vessel ( 2 ) at junction ( 6 ) and administrated to the patient ( 4 ). Optionally, there is additional length of tubing between column ( 3 ) and junction ( 6 ) to correct for the half lives of intermediate daughter isotopes. 
           [0045]      FIG. 2  depicts the decay chain of  225 Ac. 
           [0046]      FIG. 3  depicts another apparatus which may be used in practicing this invention. 
           [0047]    An eluent passes from a vessel ( 7 ) through an ion exchange column ( 3 ) where a daughter radioisotope is stripped from that column. 
           [0048]    The eluent containing the isotope is mixed with a liquid from a vessel ( 1 ) containing a targeting moiety, so that the isotope becomes bound to the targeting moiety. The resulting fluid is mixed with infusion liquid from vessel ( 2 ) at junction ( 6 ) and administered to the patient ( 4 ). Optionally the eluent containing the isotope may be lead through an additional length of tubing ( 5 ) to correct for the half-life of intermediate daughter isotopes. 
           [0049]      FIG. 4  shows the elution behavior of  213 Bi at several HCl and HI concentrations. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    An important aspect of the present invention is that the radioconjugate can be made or quasi be made at the site of therapy. Due to the decay sequence which results mainly in α- and/or β-radiation it has become possible that no protection against radiation is necessary. This is extremely useful, because due to the absence of gamma radiation it has become possible that the conjugation can be done at the bed side or in a nearby nuclear medicine lab without the necessity to apply radiation shielding or isolation of the patient or the equipment. This is not only preferable from the point of radiation danger but is also gives advantages for the availability of the short-living isotope. This isotope can be prepared in the neighborhood of the patient allowing for a rapid administration and prevention of loss of therapeutic action caused by the rapid decay of the isotope. In this way it has become possible to use short lived radioisotopes for therapy. 
         [0051]    An ion exchange column or another appropriate substrate filled with the long lived isotope can be placed at or near the bedside, for instance, where the short-lived isotope can be eluted by washing the substrate with a suitable solution. After elution the short lived isotope is coupled to the targeting moiety and (optionally together with an infusion solution) the conjugate can be administered. This can all be done in a continuous mode with an apparatus according to the invention as shown in  FIG. 1  or  FIG. 3 , or in an intermittent mode by using ordinary laboratory glassware. 
         [0052]    Of course it may also be made possible to add the targeting moiety to the eluting solution so that the coupling takes place in the column. 
         [0053]    As noted above, the targeting moiety of the radioconjugate and the target moiety of the pathological site constitute a specific binding pair. The expression “specific binding pair”, as used herein refers to complementary substances that selectively recognize and interact with one another, to the substantial exclusion of other substances. Representative examples of specific binding pairs include, without limitation, antibodies (which may be monoclonal or polyclonal immunoglobulins or immunoreactive fragments thereof) and cell-associated targets, such as membrane-bound proteins or glycoprotein, including cell surface antigens of either host cell or viral original or histocompatibility antigens; ligand or ligand fragment and receptor; hormone-receptors and lectin-receptors. 
         [0054]    The invention is described hereinbelow with particular emphasis on the use of antibodies as the targeting moiety of the radioimmunoconjugate of the invention. It should be understood, however, that any of the other targeting moieties described above may also be used to effect site directed delivery of the radioconjugate. 
         [0055]    This invention primarily addresses the use of the shortest-lived isotope from the list mentioned above,  213 Bi. The invention enables the person skilled in the art to milk this isotope by a continuous or an intermittent extraction-method from one of its precursors,  225 Ac, at the bed-side of the patient, or in the nearest by hospital laboratory facility, to link the  213 Bi in a continuous or an intermittent manner onto the targeting moiety, to either or not mix the conjugate solution with an infusion liquid and to administer this mixture intravenously to the patient for example as is schematically pictured in  FIG. 1 . 
         [0056]    At first sight this procedure might seem extremely wasteful, because  225 Ac, itself being an α-emitting isotope, produces three potentially therapeutically useful α-particles before yielding the  213 Bi-isotope, as is shown in  FIG. 2 . However, the source material for  225 Ac,  229 Th, and thereby also the  225 Ac itself, can be made available at sufficiently low cost to allow it to be used in the proposed manner on economically justifiable terms. 
         [0057]    The use of  213 Bi is not only preferable from a viewpoint of radiation hazards. It is also preferable because no gaseous isotopes occur in the decay sequence of its precursors. This is advantageous over the use of other isotopes which have a decay with a gaseous isotope which necessitates the handling and reaction environment to be air-tight. Milking, conjugation and administration of  213 Bi are not hampered by the necessity for having air-tight conditions and the reactions can be done under normal conditions. 
         [0058]    The targeting moiety may preferably be a monoclonal antibody, or a fragment or a derivative thereof. Preferably such an antibody is a human or a humanized antibody to prevent immunologic reactions to the antibody. Non-human antibodies are mostly of murine origin. These, like all other foreign proteins, are highly immunogenic in man. The phenomenon of HAMA and of HAHA, is well known in the field and severely limits the use of mouse derived antibodies in diagnostic and especially in therapeutic applications in human beings. A single application of a murine antibody is usually sufficient to mount an immune response that will prevent subsequent applications to be effective. 
         [0059]    Of course fragments and/or derivatives of the targeting moieties can also be used, as long as they retain a substantial amount of target specificity. Thus, for this invention it should be understood that where a targeting moiety is mentioned one should also consider a fragment or a derivative thereof, as well as any other appropriate site-selective moiety, as part of the invention. 
         [0060]    Preferably antibodies are directed against tumor associated antigens, such as CEA (Carcino-embryonic antigen), AFP (alpha-foetoprotein), FHAP (fast homoarginine-sensitive alkaline phosphatase), p97 (melanome specific), and EL-1 (elongation factor 1). 
         [0061]    Another preferable targeting moiety is formed by a ligand for a cell surface receptor or a fragment or derivative of such a ligand. Examples of such ligands are agonists and/or antagonists of pharmacologically active receptors, but also T cell epitopes which can bind to the T cell receptor are preferred. 
         [0062]    Another aspect of the invention provides a method for treating numerous patients with one ion exchange column loaded with isotope. The amount of isotope loaded depends on the number of patients to be treated. The wanted isotope can be eluted from the column intermittently, with suitable intervals depending on the half-lives in the decay chain. 
         [0063]    With related tumours or infectious organisms the same targeting moiety (or mix of targeting moieties) may be used for various patients. For unrelated diseases there must be a means for changing the targeting moiety preparation. 
         [0064]    The coupling of the isotope to the targeting moiety can be done in any suitable way, as long as the targeting specificity of the targeting moiety is not hampered to a substantial amount. 
         [0065]    Preferably the coupling will be done through one of the now many known chelating agents. As already disclosed, it may be advantageous to couple the isotopes to a carrier, such as HSA, which of course can also be done through chelating agents. The advantage of a carrier is that a large number of radioisotopes can be brought to the target cell. Since it is assumed that several α-particles are necessary for the destruction of one target cell an increase in the number of isotopes in the direct neighborhood of the target cell is preferable. 
         [0066]    The invention also provides a conjugate as produced by the method of the invention, as well as a pharmaceutical formulation comprising such a conjugate. 
         [0067]    A method is provided for producing the conjugate of a targeting moiety and a radioisotope and administering it to the patient without delay or any necessary actions of the therapist. 
         [0068]    Another aspect of the invention provides an apparatus for carrying out site directed therapy or imaging. 
         [0069]    The simplest way to describe the method and apparatus, subject of this invention, with reference to  FIG. 1 , is as follows: 
         [0070]    A capillary column contains, by means of example, twice the amount of precursor- 225 Ac needed for a single patient dose of  213 Bi. Example: in a case the patient dose corresponds with 30 mCi (equals 2.10 −9  g) of  213 Bi over a 10 day period, the capillary column ( 3 ) will contain 200 μCi of  225 Ac (equals 4.10 −9  g). 
         [0071]    The  225 Ac is present in a 3+ form on a suitable ion exchange substrate. Upon its (continuously occurring) decay it is stripped from the column by a certain overdose of the eluent in flask ( 1 ) containing the appropriate targeting moiety capable of binding the isotope. The binding part of the targeting moiety and other chemical equilibrium conditions of the eluent-ion exchange system are chosen such that the  213 Bi, for all practical purposes, quantitatively binds to the targeting moiety. The immediate daughter of  225 Ac,  221 Fr has a radioactive decay halflife of 4.8 minutes. It is this isotope which acts via the very short-lived  217 At as the direct precursor of  113 Bi. In case the  221 Fr is not retained by itself or in the ion exchange substrate, the delaying effect of the  221 Fr-halflife causes the need of a certain period of time between the decay of  225 Ac at and its stripping from the capillary column and the binding of the 213Bi onto the targeting moieties. The optimum value for such a delay is somewhere between the halflife of the  221 Fr and the halflife of the  213 Bi isotopes. 
         [0072]    This delay can be effected by the length of tubing between the capillary ( 3 ) and the patient ( 4 ), if necessary enhanced by an extra length of intermediate tubing, as indicated in  FIG. 1  as ( 5 ). The infusion liquid from flask ( 2 ) enters the patient, it is mixed with the isotope-containing eluate from column ( 3 ), as indicated as junction ( 6 ) in  FIG. 1 . 
         [0073]    In order to obtain optimal stripping and conjugation conditions in the capillary column ( 3 ), it may be that the composition of the eluent in flask ( 1 ) is not optimal (for example its pH-value) for administration to the patient. Presuming that the volume rate of infusion liquid is an order of magnitude higher than of the eluate liquid, this can easily be countered for by a compensating off-balance (buffered) pH-value of the infusion liquid. 
         [0074]    It is also possible that the binding of the targeting moiety is hampered by the physico-chemical properties of the eluent. Therefore, an other embodiment of the invention is represented in  FIG. 3  where an eluens is lead from a vessel ( 7 ) through an ion exchange column ( 3 ) so that a radioisotope is stripped from that column. The eluens containing the isotope is mixed with a liquid from a vessel ( 1 ) containing a targeting moiety, so that the isotope is bound to the targeting moiety. The resulting fluid is mixed with infusion liquid from vessel ( 2 ) at junction ( 6 ) and administered to the patient ( 4 ). Optionally the eluens containing the isotope may be lead through an additional length of tubing ( 5 ) to correct for the half-life of intermediate daughter isotopes. 
         [0075]    What the invention enables in terms of the development and the clinical use of α-radioimmuno therapy, in this case using  213 Bi as the active cell killing agent is:
       “single patient kits” in the form of precursor with a halflife that is logistically manageable regarding:   minimization of active material loss by radio active decay during operations like packaging, transport, etc.,   safety in transportation over long distances and in handling in hospitals,   applicability in practice on a large scale in many hospitals without need for special precautions, regarding:   the handling of the material and the application procedures regarding the treatment of patients, all without complicated monitoring equipment,   collection and handling facilities for (urinous) waste,   maximal (and in case of continuous extraction, almost total) use of the  213 Bi after it is generated from the precursor isotope,   maximum flexibility in dose administration by the possibility of changing treatment time, allowing for a minimum range of single patient kit precursor concentration standards.       
 
         [0084]    All these aspects then pertain precisely to fields where the short-range α-particles are most suited for their potential therapeutic uses like:
       micrometastases (of less than 1 mm diameter) of various cancers,   cellular cancers like leukemias and   also, certain kinds of very localized autoimmune diseases, all of which can essentially be directly addressed either by the blood circulation system or locally without the need for slow diffusion processes of the antibody ligand-isotope complexes through intercellular space in order to find their ultimate destination.       
 
         [0088]    The radioconjugate of the invention may be used not only for treating cancer, immune or infectious diseases, but also for the treatment of inflammatory conditions, such as rheumatoid arthritis, arteritis, endometritis or the like, as well as for the removal of new growths or other abnormal tissue. These radioconjugates are also useful for the detection of pathological, extra-cellular structures, including, without limitation, fibrin or thrombin within a coronary vessel at a site of infarct, cholesterol at a site of atherosclerotic plaque, calcification in diseased vessels, amyloid plaque at disease sites in the brain, or the like. 
         [0089]    A special advantage of intermittent administration of the therapeutic radioconjugates is the advantage which occurs by dose fractionation. Statistically it is possible to calculate the dose needed to kill 99.9% of the tumor cells with a dose of radioconjugate: assuming that a leukemic (monocellular, blood and bone marrow) tumor load of 1 kg exists, which is roughly equal to 10 12  cells, and that 10 α-particles are needed to kill a cell (6 MeV), then 10 13  α-particles would be needed, which corresponds with 50 mCi  213 Bi. Thus for a single dose, which would kill 99.9% of the tumor cells 50 mCi  213 Bi would be needed. 
         [0090]    The “dose versus survival” relation for this cell morphology with 6 MeV a-particles can be derived from the formula D/Do=−1n S, in which S survival fraction, D=dose administered and D o =reference dose for 37% survival. From this formula the following Table of values can be calculated: 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Dose versus kill ratio for tumor cells. The numbers are the number 
               
               
                 of α-particles necessary to kill the given % of tumor cells. 
               
               
                 In case A 600 rad are necessary to obtain a 99% kill ratio. In 
               
               
                 case B 2000 rad is assumed necessary for the same effect. 
               
             
          
           
               
                 Cell kill in % 
                 Case A 
                 Case B 
               
               
                   
               
             
          
           
               
                 0 
                 0 
                 0 
               
               
                 1 
                 0.015 
                 0.05 
               
               
                 10 
                 0.15 
                 0.5 
               
               
                 40 
                 0.7 
                 2 
               
               
                 50 
                 1 
                 3 
               
               
                 60 
                 1.3 
                 4 
               
               
                 70 
                 1.5 
                 5 
               
               
                 90 
                 3 
                 10 
               
               
                 99 
                 6 
                 20 
               
               
                 99.9 
                 9 
                 30 
               
               
                 99.99 
                 12 
                 40 
               
               
                 99.999 
                 15 
                 50 
               
               
                   
               
             
          
         
       
     
         [0091]    From this Table the effects of an intermittent, dose fractioned, administration can be read: 
         [0092]    The effect of cell survival of successive doses of 5 mCi  213 Bi in case A is as follows:
       the first dose of 5 mCi equals 1 α-particle per cell, which gives 50% survival, which means that 0.5·10 12  cells remain;   the second dose of 5 mCi equals 2 α-particles per cell, which gives 20% survival, which means that 0.1·10 12  cells remain;   the third dose of 5 mCi equals 10 α-particles per cell, which gives 0.1% survival, which means that 0.1·10 9  cells remain;   the fourth dose of 5 mCi equals 10,000 α-particles per cell, which means a total kill.       
 
         [0097]    Thus it can be shown that by intermittent dosing a total dose of 4 times 5=20 mCi  213 Bi is sufficient to give a total kill of the tumor cells. For clarity the effects of intermediate tumor growth and maximization of the number of targeting moieties on the tumor cells have been omitted. Nevertheless, it is clear that by intermittent administration the total load of radioactive material can be kept smaller. 
         [0098]    Even in case B, which has a more unfavourable dose versus survival rate, and advantageous effect is realized: 
         [0099]    1st dose→1 α/cell→75% survival→0.75·10 12  cells 
         [0100]    2nd dose→1.3 α/cell→70% survival→0.50·10 12  cells 
         [0101]    3rd dose→2 α/cell→60% survival→0.30·10 12  cells 
         [0102]    4th dose→3 α/cell→50% survival→0.15·10 12  cells 
         [0103]    5th dose→6 α/cell→25% survival→0.04·10 12  cells 
         [0104]    6th dose→25 α/cell→0.3% survival→0.1·10 9  cells 
         [0105]    7th dose→10,000 α/cell—total kill after 35 mCi. 
         [0106]    There are two ways presently known to obtain 229Th as a precursor for the  225 Ac-source-isotope: 
         [0107]    from stockpiled  233 U, by its natural a-decay. Batches of  233 U were made in nuclear breeder reactors about 30 years ago, but never used as nuclear fuel. 
         [0108]    Some of the  233 U was separated from the bulk  233 Th, from which it was made, so that the now available  229 Th can be obtained in highly pure form. 
         [0109]    by high neutron flux irradiation from natural  226 Ra, with  227 Ac as an intermediate product. Further irradiation of this  227 Ac yields roughly equal amounts of  229 Th and  228 Th, the latter with much shorter halflife (2 years) than the  229 Th. On the one hand this complicates the extraction of  225 Ac considerably, but in properly equipped installations it may on the other hand yield  224 Ra, an α-emitter with a 3.7 day halflife. When the Ra is properly isolated, it may be used as a source for  212 Pb. 
         [0110]    The 10.5 hour halflife of  212 Pb will cause considerable complications in handling. However, when these are properly taken care of, one may envisage to use the  212 Pb-isotope in the same manner as the  225 Ac in this invention as a bed-side source of  212 Bi, which for all practical purposes acts as an α-emitter with a halflife of 1.0 hour. 
       EXAMPLES 
     Example 1 
       [0111]    The separation chemistry of the various radioactive elements mentioned in the text before has been sorted out decades-ago and is well-documented in the public literature. Examples are references (3) and (4).  225 Ac can be separated from  229 Th on a Dowex 50 ion exchanger by stripping with 4N HNO 3 . After evaporation of the acid, the  225 Ac can be dissolved again in 0.5N HNO 3  in a fixed concentration and absorbed in the appropriate amount on Dowex 50, which then becomes the material in the mini-column ( 3 ) of  FIG. 3 . 
       Example 2 
       [0112]    0.68±0.07 mCi of  225 Ac was obtained from the European Joint Research Centre. This was loaded on a MP-50 cation exchange resin (Bio-Rad). The formed  213 Bi was eluted with a mixture of 50:50 10% NH 4 Ac:MeOH with a pH of 6.75. An autoburet was used to deliver 35 μl of eluant per minute; alternatively, manual elution was done at 50 μl amounts of eluant per minute. 
         [0113]    In a few experiments, it was necessary to purify the  213 Bi. This was accomplished by heating the eluant to dryness in a 10 ml beaker containing 0.5 ml of conc. HNO 3 . After evaporation under an IR lamp, the bismuth activity was transferred to a column of MP-50 resin (2×30 cm, pre-equilbrated with 0.1M HNO 3 ). The resin was washed with 0.2 ml H 2 O. Then the  213 Bi was eluted with 0.5 ml of Hcl and HI. Various concentrations of Hcl and HI have been tried.  FIG. 4  shows the elution patterns for  213 Bi. In all cases, the elution is rapid and quantitative. All of the isotope can be obtained within 5 to 10 minutes after the start of the elution. 
         [0114]    Elution of  213 Bi from  225 Ac can also be done with generators containing 25 mCi of  225 Ac, as is now the routine practice at Sloan-Kettering in New York City for the ongoing clinical phase-1 trial (the first one ever with systemically administered α-emitting isotope,  213 Bi) against acute myelogenous leukemia. The procedure used is simple: Elution with 0.1 Mol HCl/HI suffices to quantitatively yield larger than 15 mCi samples of  213 Bi free of  225 Ac and of  221 Fr. 
       Example 3 
       [0115]    Radiolabeling was done by adding enough 3M NH 4 Ac to the  213 Bi stock to achieve pH 4.0-5.0. Then 53 μl or 106 μl of a 4.7 mg/ml solution of monoclonal antibody B3 coupled with the chelator CHX-DTPA (cyclohexyldiethylenetriaminepenta acetic acid) according to the method described in (5) were gently mixed into the solution. After a fifteen minute reaction time, 1.5 μl of 0.1M EDTA were added. The solution was transferred to a 1 ml syringe with 0.2 ml wash. The solution was then injected into the HPLC (high pressure liquid chromatography) having a TSK 3000 column. The buffer was 0.02 M MES/Cl −  (MES=morpholino ethane sulfonic acid), 0.15 M NaCl, pH 6.5. Elution of the B3 antibody occurred at 7.5 minutes. The amount of  213 Bi incorporated into the antibody was monitored with an in-line radiochemical detector (Beckman). All activity measurements of  213 Bi were corrected for decay (t 1/2 =45.6 min). Results are depicted in Table 2. Activities of  225 Ac,  221 Fr or  217 At were not detectable in any of the  213 Bi elution products. 
         [0116]    Radiolabeling can also be carried out using 15 mCi samples of  213 Bi with a 1000-fold overdose of HuM195 monoclonal antibody, coupled with a further overdose of chelator CHX.A.DTPA, basically according to the method described in (5) and (6). This is the procedure now employed at Sloan-Kettering in New York City. After 10 minutes reaction time, the product is separated from remaining free  213 Bi using HPLC with a TSK 3000 column. Less than 5% of the  213 Bi is lost in remaining free isotope. Due to decay during the delay caused by these procedures, about 10 mCi of antibody-bound  213 Bi is available for therapy. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Results of radiolabeling experiments incorporating 
               
               
                 213Bi into mAb B3-CHX-DTPA 
               
             
          
           
               
                   
                   
                   
                   
                   213 Bi recovered 
               
               
                   
                 Acid 
                 Vol. acid μl 
                 μg mAb 
                 in mAb (%) 
               
               
                   
                   
               
               
                   
                 2M HCl 
                 210 
                 250 
                 43.4 (37%) 
               
               
                   
                 2M HCl 
                 210 
                 500 
                 45.0 (25%) 
               
               
                   
                 0.1M HCl 
                 500 
                 500 
                 3.4 (7%) 
               
               
                   
                 0.1M HI 
               
               
                   
                   
               
             
          
         
       
     
         [0117]    Although targeting via coupling of radioisotope to a monoclonal antibody is exemplified in Example 3, the invention is not limited to targeting in this way. Various peptides are known to bind their cognate receptors with high affinity and thus would be suitable ligands for conjugation to the radioisotopes of the invention. Receptors are plasma membrane proteins which bind molecules, such as growth factors, hormones, and neurotransmitters. Tumors develop from particular cell types which express certain subsets of these receptors. Taking advantage of this binding affinity between receptor and ligand enables target-specific delivery of the radionucleotide. Representative examples of receptor-ligand pairs are set forth below: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 RECEPTOR 
                 LIGAND 
               
               
                   
                   
               
             
             
               
                   
                 epidermal growth, 
                 epidermal growth factor 
               
               
                   
                 factor receptor 
                 (53 amino acids) 
               
               
                   
                 platelet derived 
                 platelet derived growth factor 
               
               
                   
                 growth factor receptor 
               
               
                   
                 insulin like growth 
                 insulin-like growth factor 
               
               
                   
                 factor receptor 
               
               
                   
                 glucagon receptor 
                 glucagon 
               
               
                   
                   
                 (29 amino acids) 
               
               
                   
                 vasopressin receptor 
                 vasopressin 
               
               
                   
                   
                 (9 amino acids) 
               
               
                   
                 thyroid stimulating 
                 thyroid stimulating hormone 
               
               
                   
                 hormone receptor 
               
               
                   
                 insulin receptor 
                 insulin 
               
               
                   
                   
               
             
          
         
       
     
         [0118]    As all of the above-listed ligands are protein molecules, the coupling chemistry set forth in Example 3 is readily adaptable for forming radioimmunoconjugates using these ligands. 
       REFERENCES 
       [0000]    
       
         (1) D. R. Fisher: “α-particle emmitters in medicine”, proceedings of a symposium held at Loews L&#39;Enfant Plaze Hotel, Washington, D.C., Sep. 21 and 22, 1989, pages 194-214, published by the American College of Nuclear Physicians. 
         (2) D. S. Wilbur: “Potential use of α-emitting radionuclides in the treatment of cancer”, Antibody, Immunoconjugates, and Radiopharmaceuticals, Volume 4, No. 1, 1991, pages 85-97, published by Mary Ann Liebert, Inc. 
         (3) T. Mitsugashira: “Preparation of traces for actinium, thorium, protactinium and uranium”, SPEY, Min. Educ. Sci. &amp; Cult., Tokyo, 9, 1984, pages 111-116. 
         (4) S. Suzuki: “Solution chemistry of light actinide elements”, Japan-US seminar on Thorium fuel reactors—proceedings, Nara, Japan, 18-22, Oct., 1982 (Tokyo, 1985) pages 137-143. 
         (5) Mirzadeh, S., Brechbiel, M. W., Atcher, R. W., Gansow, O. A., Bioconjugate Chem., Volum 1, 1990, 59-65. 
         (6) McDevitt, M. R., Scheinberg, D. A., Brechbiel, M. W. et al., “Alpha Particle Emitting Bismuth-213 Cyclohexylbenzyl DTPA Constructs of Monoclonal Antibodies for Therapy of Cancer”, Cancer Res., 1998 (in press)