Targeted radiotherapy of bone malignancies.

The severe pain associated with many disorders affecting bone account for a large proportion of cases of patient morbidity, due to the encumbrance of mobility and therefore, compromised quality of life. Skeletal metastasis is one such condition, which generally complicates the treatment of the primary cancers such as that of the breast, prostate and lung - causing intense pain and eventually even mortality. This paper presents examples of various approaches explored and proposed in the ongoing search to identify better radiopharmaceuticals for the treatment of bone disorders such as metastases. The primary objective of these developments is to alleviate the debilitating pain commonly associated with bone lesions. The efficacy of a radiotherapeutic agent intended for the treatment of diseased bone is particularly dependent on the radiation dose to the tumor cells and on the extent to which suppression of bone marrow or other critical organs can be avoided. Therefore, the design rationale requires careful consideration of the choice radionuclide and especially ensuring that the drug selectively targets the lesion or tumor site. The options pursued include the use of radioisotopes with an intrinsic affinity for bone, such as (89)Sr or (223)Ra, or the design of bone-seeking ligands, such as phosphonates, to selectively deliver the radionuclide to the target, e.g. [(153)Sm]Sm-EDTMP. A combination of the above may too be possible, where the bone seeking ligand facilitates the selective accumulation of a radionuclide, which by itself is also bone homing. In terms of therapeutic application radionuclides with various decay modes are proposed, including beta (-) emitters: (153)Sm, (89)Sr, (186)Re, (188)Re, (32)P, (177)Lu and (170)Tm; alpha (α) emitters: (223)Ra and (225)Ra; and Auger or conversion electron emitter: (117)mSn. From a purely diagnostic perspective, the radioisotopes used for imaging include the well known photon emitting (99)mTc, and positron emitters (18)F and (68)Ga. The current status in the development and application of internal radiotherapy for the palliative treatment of bone pain will be discussed, summarizing the progress made and challenges encountered in the process to realizing an effective drug candidate.

Blood plasma model predictions for the proposed bone-seeking radiopharmaceutical [(117m)Sn]Sn(IV)-N,N',N'-trimethylenephosphonate-poly(ethyleneimine).

In an attempt to elucidate the in vivo stability of the prospective radiopharmaceutical [(117m)Sn]Sn(IV)-PEI-MP, where PEI-MP stands for N,N',N'-trimethylenephosphonate-polyethyleneimine, glass electrode potentiometry was used to determine the stability constants of the Sn(4+) ion as complexed with a variety of physiological amino acids. In addition, linear free energy relationship (LFER) correlation plots were used to extrapolate the constants of the major blood plasma ligands, based on data from Cu(2+), Pb(2+), and Zn(2+). In so doing, a thermodynamic model of blood plasma was established for Sn(4+) from which the complexation tendencies of Sn(4+) were predicted in the event of the intravenous administration of such a drug. It was found that the Sn(IV)-PEI-MP could succumb to competition by the glutamine amino acid, which forms more stable complex(es), whilst the PEI-MP gets taken up largely by Ca(2+). Also, this study shows the value of the in vitro experiments and modeling performed for radiopharmaceutical research and for attempts to reduce the number of animal experiments.

Hydroxyapatite chemisorption of N,N',N'-trimethylenephosphonate-poly(ethyleneimine) (PEI-MP) combined with Sn2+ or Sn4+.

To study the mechanism by which proposed bone-seeking radiopharmaceuticals consisting of tin (as l17mSn) and N,N',N'-trimethylenephosphonate-poly(ethyleneimine) (PEI-MP) are taken up and accumulated in bone tissue, the adsorption of Sn2+, Sn4+, and phosphonate polymer PEI-MP on hydroxyapatite was measured in vitro. Hydroxyapatite is the main mineral phase of bone; therefore, by determining the affinity of the metal ions and the ligand-and hence their complexes-for hydroxyapatite, the extent to which the radiopharmaceutical will be adsorbed can be predicted. The adsorption of the tin-phosphonate complexes and the two individual components to the solid phase was measured by inductively coupled plasma optical emission spectroscopy at physiological pH, at room temperature, and for a period of 48 h. The tin complexes and free ligand exhibited unique binding affinities. By varying the oxidation state of the metal ion (Sn2+, and Sn4+) and the size of the polyphosphonate ligand (using 10-30 and 30-50 kDa fractions), the adsorption characteristics of the individual components could be adapted. The tin-PEI-MP combination that showed the most favorable adsorption behavior was that of Sn2+ and PEI-MP(10-30 kDa), which had a maximum adsorption capacity of 2.21 +/- 0.14 micromol m(-2) and an affinity of 9.8 +/- 4.0 dm3 mmol(-1) with respect to the ligand and 2.30 +/- 0.07 micromol m(-2) and 26.6 +/- 6.1 dm3 mmol(-1) for the metal ion-as derived from the Langmuir adsorption model. The ligand showed enhanced adsorption when complexed with tin. This research provided a preliminary indication that the tin-PEI-MP combination could be favorable to other bone-seeking radiopharmaceuticals since the maximum adsorption capacities are comparable while PEI-MP offers the opportunity of using the enhanced permeability and retention effect for enhanced tumor accumulation.

Lanthanide(III) complexes of bis(phosphonate) monoamide analogues of DOTA: bone-seeking agents for imaging and therapy.

Lanthanide complexes of DOTA derivatives 2a (BPAMD) and 2b (BPAPD), having a monoamide pendant arm with a bis(phosphonate) moiety, were comparatively tested for application in MRI, radiotherapy, and bone pain palliation. (1)H, (31)P, and (17)O NMR spectroscopy show that they are nine-coordinated, with one water molecule in the first coordination sphere of the Ln(III) ion. The bis(phosphonate) moieties are not coordinated to the lanthanide and predominantly mono- and diprotonated at physiological pH. The parameters governing the longitudinal relaxivities of the Gd complexes are similar to those of other monoamides of DOTA reported in the literature. Upon adsorption on hydroxyapatite, the relaxivities at 20 MHz and 25 degrees C of Gd-2a and Gd-2b were 22.1 and 11 s(-1) mM(-1), respectively. An in vivo gamma-ray imaging study showed that the (177)Lu complexes of 2a and 2b have a high affinity for bones, particularly for growth plates and teeth with a prolonged retention.

Comparison of the predicted in vivo behaviour of the Sn(II)-APDDMP complex and the results as studied in a rodent model.

In a quest for more effective radiopharmaceuticals for pain palliation of metastatic bone cancer, this paper relates results obtained with ((117m)Sn labelled) Sn(II) complexed to the bone seeking bisphosphonate, N,N-dimethylenephosphonate-1-hydroxy-3-aminopropylidenediphosphonate (APDDMP). APDDMP is synthesised from the known bone cancer pain palliation agent 1-hydroxy-3-aminopropylidenediphosphonate (APD, Pamindronate). This work is performed to utilise the idea that the low bone marrow radio toxicity of (117m)Sn could afford a highly effective radiopharmaceutical in pain palliation but also in the curative treatment of bone metastasis. Complex-formation constants of APDDMP with the important blood plasma metal-ions, Ca(2+), Mg(2+), Zn(2+) as well as the added metal ion, Sn(2+) were measured by glass electrode potentiometry at 25 degrees C and I = 150 mM. Blood plasma models were constructed using the computer code ECCLES and the results compared with those gathered from tests on a rodent model. The ((117m)Sn-labelled) Sn(II)-APDDMP complex was found to have only some liver and bone uptake although a high trabecular to normal bone ratio was recorded. From the blood plasma model this was shown to be primarily due to the high affinity of APDDMP for Ca(II) causing some of the Sn(II)-APDDMP complex to dissociate. High kidney uptake and excretion as well as high bladder uptake was recorded which was shown to be due to the dissociation of the Sn(II)-APDDMP complex in blood plasma. Animal model observations could be explained by the blood plasma modelling.

Biodistribution and pharmacokinetics of variously molecular sized 117mSn(II)-polyethyleneiminomethyl phosphonate complexes in the normal primate model as potential selective therapeutic bone agents.

In the search for a cure for metastatic bone cancer, 117mSn with its conversion electrons and low energy photons both of discrete energies shows little bone marrow toxicity, providing the opportunity to increase the administered dose. Selective accumulation in lesions would capitalise on this advantage. The 10-30 kDa fraction of the water-soluble polymer polyethyleneimine, functionalised with methyl phosphonate groups (PEI-MP) and labelled with 99mTc, has shown selective uptake into bone tumours. Furthermore using speciation calculations it was predicted that the Sn(II)-PEI-MP complex would remain intact in the blood plasma. Because of this positive indication animal experiments were carried out to test this prediction. This paper relates the labelling, biodistribution and pharmacokinetics of various fractions of 117mSn-(II) PEI-MP in the normal primate model, and points to promising therapeutic possibilities.