Bone Seeking Technetium 99m Complex

It has been known for some time that phosphates, including long chain linear polyphosphates, when introduced into the blood stream of mammals will selectively seek out and collect in the bone or skeletal structure. Pro. Soc. Exp. Biol Med. Volume 100, pages 53-55 (1959); Journal of Labelled Compounds, April-June 1970, Vol. VI, No. 2, pages 166-173; Journal of Nuclear Medicine, Vol. 11, No. 6, pages 380-381, 1970; Journal of Nuclear Medicine, Vol. 1, No. 1, January 1960, pages 1-13. In these cases a phosphorous atom or atoms of the phosphate are radioactive, i.e. .sup.32 P.

It has also been known for some time that technetium-99m (.sup.99m Tc) is a preferred radionuclide for radioactively scanning organs because of its short half life and because it radiates gamma rays which can be easily measured, compared, for example, to beta rays. See Radiology, Vol. 99, April 1971, pages 192-196.

It has also been known for some time to use divalent stannous tin (Sn.sup.+.sup.+) in the form of stannous chloride, or divalent iron (Fe.sup.+.sup.+) or reduced zirconium to bind radioactive .sup.99m Tc to carriers, such as chelating agents, red blood cells, albumin and other proteins, which selectively seek out certain organs of the body, in order to carry the .sup.99m Tc with them to such organs of the body where it is concentrated, whereby such organ can be radioactively scanned or imaged for diagnostic or other purposes, e.g. radioactive treatment of a pathological condition. See Journal of Nuclear Medicine, Vol. 11, No. 12, 1970, page 761; Journal of Nuclear Medicine, Vol. 12, No. 1, 1971, pages 22-24; Journal of Nuclear Medicine, Vol. 13, No. 2, 1972, pages 180-181; Journal of Nuclear Medicine, Vol. 12, No. 5, May 1971, pages 204-211; Radiology, Vol. 102, January 1972, pages 185-196; Journal of Nuclear Medicine, Vol. 13, No. 1, 1972, pages 58-65.

Also, it has been suggested to label a stannous compound with .sup.99m Tc for radioactively imaging bone marrow, Journal of Nuclear Medicine, Vol. 11, 1970, pages 365-366.

It has also been known for some time that the stannous ion Sn.sup.+.sup.+ forms soluble complexes with long chain polyphosphates, Journal Inorganic Nuc. Chem., Vol. 28, 1966, pages 493-502.

It has been suggested to employ the aforesaid .sup.99m Tc for radioactively scanning the skeletal bone structure of mammals by complexing or binding it to tripolyphosphate carrier by use of the aforesaid stannous ion as a binding agent in order for such phosphate to selectively carry the .sup.99m Tc to, and concentrate it in, the skeletal bone structure upon in vivo intravenous administration for subsequent radioactive scanning or imaging the skeletal structure. Radiology, Vol. 99, April 1971, pages 192-196. The use of .sup.99m Tc in this manner is alleged to have certain advantages over the use of strontium, e.g. .sup.85 Sr, as the radioactive label which has been used for radioactive bone scanning in the past. These advantages are those which are inherent in .sup.99m Tc, i.e. short half life and pure, near optimal energy gamma rays. However, the bone uptake (the percent of the total dosage which becomes concentrated in the skeletal structure within a certain time after in vivo intravenous administration) of such .sup.99m Tc-containing complex and the ratios of such bone uptake of the .sup.99m Tc by the other organs of the body (the higher these ratios the better), i.e. radioactive contrast, are not nearly as high as with radioactive strontium.

It has been discovered that if the phosphate moiety of the .sup.99m Tc-stannous-phosphate complex, which has been suggested in the aforesaid Radiology publication, comprises pyrophosphate (P.sub.2 O.sub.7.sup..sup.-4) (which is a linear polyphosphate moiety of molecular weight less than 300), the bone uptake, bone/blood ratio, bone/liver ratio, bone/G.I. ratio and bone/kidneys ratio are substantially increased.

It has also been discovered that optimum results are achieved if such phosphate moiety contains no more than about 15 to 20 or 25%, preferably no more than 5 to 10% and more preferably no more than 5% (less than 5% is the most preferred), by weight of linear or branched chain polyphosphate (formula P.sub.n O.sub.3n.sub.+1.sup..sup.-(n.sup.+2)) of molecular weight greater than that of pyrophosphate.

Maximum bone/liver ratios are achieved when the .sup.99m Tc-Sn.sup.+.sup.+-pyrophosphate complex is administered to the mammal in relatively small dosages of substantially less than 20 or 25, preferably substantially less than 8 or 10 and still more preferably less than 5 or 6 (between 0.01 or 0.10 and 3 and even less provide excellent results), milligrams pyrophosphate moiety per kilogram of body weight of the mammal.

The term "phosphate moiety" as used herein refers to the phosphorus and oxygen atoms only of the phosphate.

The presence of polyphosphates of formula P.sub.n O.sub.3n.sub.+1.sup..sup.-(n.sup.+2) and molecular weight greater than that of pyrophosphate seems to reduce bone take-up and the aforesaid ratios, as compared to complexes without such higher molecular weight polyphosphates. However, as aforesaid, some of such higher molecular weight polyphosphates can be tolerated, preferably not more than about 15% to 20% or 25%, more preferably no more than 5% to 10% and still more preferably not more than 5% (less than 5% is the most preferred), by weight of the total phosphate moiety.

Where the pyrophosphate does not constitute 100% of the phosphate moiety of the .sup.99m Tc-Sn.sup.+.sup.+-phosphate complex, the rest of the phosphate moiety is preferably a ring phosphate of formula p.sub.n O.sub.3n.sup..sup.-n (with n preferably being 3 which is trimetaphosphate) and/or ortho phosphate, and preferably a ring phosphate only, although the aforesaid limited amounts of higher molecular weight linear polyphosphates can be tolerated.

The complex is made from a water soluble alkali metal (preferably sodium) or ammonium salt or acid salt of the pyrophosphate, e.g. sodium pyrophosphate.

Preferably, the sodium pyrophosphate is admixed with a stannous salt, e.g. SnCl.sub.2 (the stannous salts of other acids which are pharmaceutically acceptable, i.e. safely intravenously administered, can be used) to form the stannous-pyrophosphate complex, the pH of which is adjusted to 3-8, preferably 5-8, by a pharmaceutically acceptable acid, such as HCl, or base, such as NaOH or Na.sub.2 CO.sub.3 or NaHCO.sub.3, followed by admixing with the stannous-pyrophosphate complex, an aqueous saline solution of radioactive sodium pertechnetate (.sup.99m Tc) to form the .sup.99m Tc-stannous-pyrophosphate complex at the time it is desired to intravenously administer the .sup.99m Tc complex. The stannous-pyrophosphate complex may be sealed in a sterile, non-pyrogenic container or vial as a solution or a lyophilized solid and shipped as a kit with the freshly generated sterile and non-pyrogenic .sup.99m Tc being added aseptically at the situs just prior to use.

The following compositions were prepared:

An aqueous solution of each of the phosphate composition samples 1 through 10 (40 mg. phosphate/1 ml. solution) were made with distilled water in which the dissolved oxygen content was reduced in a conventional manner by bubbling through such water gaseous nitrogen for a period of two hours. The water and phosphates were mixed to form the solutions in a nitrogen atmosphere and in a nitrogen flushed container. The reason for this is to reduce oxidation of the divalent Sn.sup.+.sup.+ to be subsequently admixed with each solution sample. However, it is not essential (but high preferred) to use nitrogen-treated water or a nitrogen atmosphere or a nitrogen-flushed container. Other known pharmaceutically acceptable conditions, which will inhibit oxidation of the Sn.sup.+.sup.+ upon subsequent mixing thereof with the phosphate solution, can be used, including the use of conventional pharmaceutically acceptable reducing agents and anti-oxidants in the products used.

Each of these solutions, samples 1 through 10, in an amount equal to 100 ml, was mixed with 0.16g of solid SnCl.sub.2. 2H.sub.2 O under a nitrogen atmosphere. The SnCl.sub.2. 2H.sub.2 O was made by adding to 84.5 mg. of metallic tin, sufficient concentrated HCl with mixing until all the tin has dissolved followed by removing excess acid and water by lyophilization (this operation also being carried out in a vacuum or in a nitrogen atmosphere and in a nitrogen flushed container to prevent oxidation of stannous to stannic). Antioxidants, which can be administered intravenously, may also be used. A stannous (Sn.sup.+.sup.+)-phosphate complex or mixture of some kind was formed in each case, the phosphate moiety of each sample corresponding to the phosphate moieties of the phosphates set forth in TABLE 2.

Sufficient aqueous solution 3N sodium hydroxide (sodium carbonate or bicarbonate can also be used), in the case of samples 1 through 7 and 9 and 10, and 3N HCl, in the case of sample 8, is then added to each sample to give a pH of 5.0 to achieve a pH suitable for subsequent intravenous in vivo administration into the body of a mammal, in this case adult mice. The pH adjustment is preferably done under a nitrogen atmosphere also.

After thorough mixing, the solutions are sterilized by passing them through a Millipore biological filter of 0.22 micron pore size under a nitrogen atmosphere. Thereafter milliliter portions of each of the sterile solutions are poured into individual sterile and non-pyrogenic storage glass vials under a nitrogen atmosphere.

In the case of each sample, vials are lyophilized by conventional freeze drying equipment under aseptic conditions to remove water. This provides a solid stannous-phosphate complex which aids in shipping and storage and which is more stable than the complex in solution.

Each vial contains 1.35 mg. SnCl.sub.2 and 40 mg. of the phosphate.

The vials can be sealed and stored until needed subsequently to form the technetium-99m-stannous-phosphate complex at the use situs.

To prepare the technetium-99m complex, 3 to 7 (5) ml. of fresh sodium pertechnetate, removed as a sterile non-pyrogenic eluate from a sterile NEN .sup.99m Tc Generator (any other source of pharmaceutically acceptable .sup.99m Tc can be used, including 99mTc generators manufactured by others than NEN), in a 0.9% saline solution is aseptically added to each vial containing the sterile and non-pyrogenic stannous-phosphate complex and the vial is swirled until a solution is obtained. In each case a technetium-99m-stannous-phosphate complex or mixture of some kind is formed in aqueous solution (8 mg. phosphate per ml solution when 5 ml of pertechnetate is used), the phosphate moiety of which corresponds to the phosphate moieties of the phosphate compounds of each sample set forth in TABLE 2.

Aseptic techniques and sterile, non-pyrogenic ingredients and containers were used at all steps, such procedures being standard to those skilled in the art.

An eleventh sample was prepared in the manner set forth above by diluting sample 8 to a concentration of 1 mg of phosphate per ml of solution. This was labeled Sample 11.

Each of the technetium-99m-stannous-phosphate complex-containing solutions is aseptically intravenously injected in vivo into a vein in the tail of adult mice (average weight 0.040 kgs) in an amount equal to between 1 and 3 mCi and a volume of 0.12 ml (8 mg. of phosphate per ml solution in samples 1 through 10 and 1 mg. phosphate per ml. solution in sample 11). Also sample 8 was injected in the same manner except that a volume of 0.015 ml was injected instead of 0.12 ml to reduce the dosage of the complex by a factor of 8. This was labeled Sample 12.

Three hours after intravenous administration, some of the mice to which each sample was administered were sacrificed and the various organs of their bodies (skeletal, liver, G.I., blood, kidneys) were counted by conventional gamma ray counting techiques to determine uptake of .sup.99m Tc by each organ and thereby determine contrast of bone uptake as compared to uptake by the other organs. As aforesaid, it is not only important to have a high bone uptake (based on total technetium-99m-dosage but it is also important that the ratio of uptake by the bone to uptake by the other organs be high.

The results are set forth in TABLE 2 below, in which the uptakes (the bone uptake figures represent the average bone uptake for the skeletal system) are in terms of percent of the total technetium-99m activity injected (corrected for radioactive decay) which has collected in the various organs indicated three hours after in vivo intravenous injection, in which the ratio amounts are computed from the uptake amounts adjusted to compensate for the different weights of the organs and the different weights of the animals by dividing each uptake amount by the ratio expressed in percentage of the weight of such organ to the weight of the animal, in which "Percent Having Phosphate Moiety M. W. Less Than 300 " refers to weight percent of the phosphate moiety based on the total phosphate moiety of the sample identified in the first horizontal column, in which the percents referred to under Phosphate Composition are weight percents of the whole phosphate moiety of the sample (as aforesaid, phosphate moiety as used herein is limited to that part of the compound or complex made up of phosphate phosphorus and oxygen atoms), in which Ortho P1 refers to the phosphate moiety of sodium orthophosphate, Pyro P2 refers to the phosphate moiety of sodium pyrophosphate, Tripoly P3 refers to the phosphate moiety of sodium tripolyphosphate, Tetrapoly P4 refers to the phosphate moiety of sodium tetrapolyphosphate, Trimeta R3 refers to the phosphate moiety of sodium trimetaphosphate, Tetrameta R4 refers to the phosphate moiety of sodium tetrametaphosphate, both trimeta and tetrametaphosphates falling within the class of cyclic or ring phosphates having the formula P.sub.3 O.sub.3n.sup..sup.-n, in which "Pentapoly And Longer Linear Chains" refers to the phosphate moiety of sodium pentapolyphosphate and longer linear (linear as used herein includes straight and branched linear phosphate chains) polyphosphates of formula P.sub.n O.sub.3n.sub.+ 1.sup..sup.-(n.sup.+2), in which "Average M.W." refers to the average molecular weight of the phosphate moiety of the sample and in which "Fraction In Raw Stock" with reference to samples 1-1, 1-2, 1-3, 1-4, 1-6, 1-7 and 1-8 refer to the normalized percent by weight of each of these samples in sample 1, which is the raw stock which is fractionated.

Conventional gamma counting techniques for measuring technetium 99m take-up in the organs are conventional gamma ray-excitable scintillation counters for radioassaying multiple samples of the organs of the sacrificed mice.

Also, conventional scanning by radioactive imaging using a gamma ray-excited scintillation or gamma camera and a dual crystal rectilinear scanner was used in vivo. In vivo scintiphotos of the total body using the Anger camera were obtained as well as rectilinear total body scans.

The figures given in TABLE 2 are average figures achieved by the aforesaid conventional counting techniques, each sample having been intravenously administered to mice followed by radioactive counting.

Following intravenous administration, the .sup.99m Tc-Sn.sup.+.sup.+-pyrophosphate complexes of the present invention are rapidly cleared from the blood by deposition in bone and excretion into urine. Thus, the technetium-99m-stannous-pyrophosphate complexes are metabolizable. The deposition of the .sup.99m Tc-stannous-pyrophosphate complexes of the invention appears to be primarily a function of the bone blood flow as well as being related to the efficiency of the bone in extracting the complex from the blood which perfuses the bones.

It was observed that the deposition of the .sup.99m Tc in the skeleton is bilaterally symmetrical with increased accumulations being present in the axial skeleton as compared to the appendicular skeleton. There is also increased deposition in the distal aspect of long bones.

Localized areas of abnormal accumulation of the radio-pharmaceutical may be seen in primary malignancies of the bone, metastatic malignanacies of the bone, acute or chronic osteomyelitis, arthritides, recent fractures, areas of ectopic calcification, Paget's disease, regional migratory osteoporosis, areas of aseptic necrosis and in general any pathological situation involving bone in which there is increased osteogenic activity or localized increased osseous blood perfusion.

The acute toxicity level in mice (LD.sub.50/30) for Sample No. 2 has been determined to be 150 mg/Kg body weight and for Sample 6 it is 800 mg/Kg and for Sample 8 it is 70 mg/Kg. Subacute toxicity studies in mice of Sample 2 have shown no signs of toxicity after 15 daily injections at dose levels as high as 63 mg/Kg body weight/day. A similar subacute study in dogs indicates no signs of toxicity at a dose level of 3.6 mg/Kg body weight/day.

It was found that samples 4 and 6 were only one fourth as toxic to mice as sample 2 and one-eighth as toxic to mice as sample 1.

The complexes of the invention have been used successfully as a skeletal imaging or scanning agent to visualize areas of altered blood flow to the bone and altered osteogenic activity, including suspected bone lesions not shown on X-ray, bone survey performed as part of a work-up in patients with known or suspected malignancy, to follow the response of metastatic or primary bone lesions to radiation therapy, metabolic bone disease, to diagnose arthritis and osteomyelitis, and to diagnose and determine healing rate of bone fractures.

The technetium-99m (.sup.99m Tc) labeling reactions involved in preparing the .sup.99m Tc-stannous-phosphate complexes of the invention depend on maintaining the tin in the reduced or stannous (Sn.sup..sup.+2) state. Oxidants present in the pertechnetate supply may adversely affect quality.

The radioactive dosage of the .sup.99m Tc complex of the invention may vary from 1 to 25 mCi (millicuries) but preferably is from 10 to 15 mCi. The dosage should preferably be substantially less than 20 or 25, preferably less than 8 or 10 and more preferably less than 5 or 6 mg. of pyrophosphate moiety per kilogram of body weight of the mammal since greater pyrophosphate dosages than this reduces the bone-liver ratio too much. Note for example the low bone-liver ratio in sample 8 where the dosage was 25 mg. pyrophosphate per Kg body weight compared to the bone-liver ratio of sample 11 where the dosage was 3.1 mg/Kg body weight.

Only trace amounts of pyrophosphate moiety in the dosage, e.g. as low as 0.001, more preferably 0.01, mg/Kg body weight, gives good bone take-up and bone-to-other organ take-up ratios.

The dosage of pyrophosphate can be kept small either by use of more dilute dosage solutions of the pure pyrophosphate, as in sample 11 or by administering smaller doses of a more concentrated complex solution, the phosphate moiety of which contains a high concentration of pyrophosphate or by more concentrated solutions of phosphate containing, in addition to the pyrophosphate, ring phosphate and/or orthophosphate which effectively dilute the pyrophosphate concentration of the dose.

It is preferred to use a .sup.99m Tc-Sn.sup.+.sup.+-phosphate solution containing between 0.1 and 40, more preferably between 0.5 and 4 or 5 mgs of pyrophosphate moiety per ml of solution.

An advantage of a complex containing a relatively large amount of ring phosphate and a smaller amount of pyrophosphate is that the ring phosphate in addition to providing excellent bone up-take and bone-to-other-organ ratios is less toxic than pyrophosphate, although pyrophosphate, alone, is still not unduly toxic.

Scanning may be commenced as early as one hour after intravenous administration and may be as long after injection as clinically useful amounts of .sup.99m Tc remain in the organ.

Another manner of making the complex of the invention is to weigh 4 mg. of SnCl.sub.2. 2H.sub.2 O and 100 mg. of sodium pyrophosphate into a flask (the flask is sterile and non-pyrogenic and is flushed with nitrogen before weighing and is kept under nitrogen during this step and for the next step). Add, under aseptic conditions, 12 ml of sterile, non-pyrogenic sodium pertechnetate in 0.9% saline solution. Shake the mixture until a solution is obtained followed by intravenous injection (preferably the pH of the mixture is aseptically adjusted to pH 3-8 before intravenous injection).

Also, the sterile stannous chloride can first be aseptically mixed with the sterile .sup.99m Tc saline solution to form a .sup.99m Tc-stannous complex, followed by adding the sterile sodium pyrophosphate under aseptic conditions to form the .sup.99m Tc-stannous-pyrophosphate, adjusting the pH to 3-8, followed by intravenous injection.

It can be seen from TABLE 2 that a .sup.99m Tc-stannous-phosphate complex in which the phosphate moiety comprises pyrophosphate and in which such moiety contains no more than 25% by weight of linear polyphosphate of molecular weight greater than that of pyrophosphate (samples 1-7, 1-8, 2, 4, 5 and 6, 8, 11 and 12) gives surprising higher bone uptake and ratio of bone uptake to other organs, as compared to orthophosphate and other polyphosphates, e.g. tripolyphosphate, tetrapolyphosphate and longer chain polyphosphates (see samples 1, 1-2 to 1-6, 7, 9 and 10).

It can also be seen by comprising samples 8, 11 and 12 in TABLE 2 that the bone to liver ratio is substantially increased by reducing the amount of pyrophosphate in the dosage administered to the mammal.

The pyrophosphate moiety of the .sup.99m Tc-stannous-phosphate complex may be from 1 or 2% or even less up to 100% by weight of the total phosphate moiety. Preferably, the pyrophosphate moiety consists of 5 or 10% or more of the total phosphate moiety, more preferably 50% or 60% or more and most preferably between 90 and 100%.

Although the stannous (Sn.sup.+.sup.+) ion is by far preferred, the divalent ferrous (Fe.sup.+.sup.+) ion in the form of ferrous ascorbate, and reduced zirconium can also be used but without as good results. All these metals can exist in a plurality of redox states.

The phosphate may be added to the solid SnCl.sub.2 as an aqueous solution, or it may be added to a solution of the SnCl.sub.2 to form the Sn.sup.+.sup.+-phosphate complex followed by adding the .sup.99m Tc solution.

Very little Sn.sup.+.sup.+ need be used to form the complex of the invention, e.g. less than 7 or 10% of the phosphate based on molecular weights.

The weight ratio of Sn.sup.+.sup.+ ion to the pyrophosphate moiety may vary over a wide range, i.e. from 10.sup..sup.-3 to 0.50, preferably 0.01 to 0.4. It is preferred that the molecular ratio of Sn.sup.+.sup.+ to pryophosphate moiety not exceed 2/1. The maximum ratio is dictated by the amount beyond which the precipitation of Sn.sup.+.sup.+ occurs. The minimum amount required is that amount necessary to bind a sufficient amount of .sup.99m Tc to the pyrophosphate to achieve good bone uptake and contrast. This can be determined by routine experiment.

The pH of the stannous-phosphate complex may be between 3 and 8.

The water used for making the complexes of the invention is distilled and is at an elevated temperature of 200.degree. F during removal of dissolved oxygen and reduction of oxidants by bubbling the nitrogen gas therethrough.

The maximum amount of .sup.99m Tc is that beyond the capacity of the Sn.sup.+.sup.+-pyrophosphate complex to bind the .sup.99m Tc. This can be determined by routine thin layer radiochromatography to determine the percent of free or unbound .sup.99m Tc in the complex. The minimum amount is dictated by that amount below which there is an insufficient amount to give good scanning of bone uptake and contrast, which also can be determined by routine experiment. Generally, the amount of .sup.99m Tc added to the Sn.sup.+.sup.+ pyrophosphate complex should be sufficient to achieve the counting rate desired by the doctor or laboratory personnel for the volume to be injected; ordinarily, as aforesaid, the activity dosage varies from 5 to 25 millicuries.

Although sodium pyrophosphates are preferred, any alkali metal, such as potassium and lithium, or ammonium can be used as the cation so long as it is pharmaceutically acceptable so that it can be safely administered intravenously. Also the acid pyrophosphates of such cations can be used.

Although in the examples given above saline water was used as the vehicle, any other vehicle which is pharmaceutically acceptable for intravenous administration can be used.

It is not intended that the invention be limited to any theory which may have been given above or to the specific examples set forth above but only by the claims appended hereto and their equivalents.