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Production, quality control, biodistribution assessment and preliminary dose evaluation of [177Lu]-tetra phenyl porphyrin complex as a possible therapeutic agent

Abstracts

Due to interesting therapeutic properties of 177Lu and tumor avidity of tetraphenyl porphyrins (TPPs), 177Lu-tetraphenyl porphyrin was developed as a possible therapeutic compound. 177Lu of 2.6-3 GBq/mg specific activity was obtained by irradiation of natural Lu2O3 sample with thermal neutron flux of 4 × 1013 n.cm-2.s-1. Tetraphenyl porphyrin was synthetized and labeled with 177Lu. Radiochemical purity of the complex was studied using Instant thin layer chromatography (ITLC) method. Stability of the complex was checked in final formulation and human serum for 48 h. The biodistribution of the labeled compound in vital organs of wild-type rats was studied up to 7 d. The absorbed dose of each human organ was calculated by medical internal radiation dose (MIRD) method. A detailed comparative pharmacokinetic study was performed for 177Lu cation and [177Lu]-TPP. The complex was prepared with a radiochemical purity: >97±1% and specific activity: 970-1000 MBq/mmol. Biodistribution data and dosimetric results showed that all tissues receive approximately an insignificant absorbed dose due to rapid excretion of the complex through the urinary tract. [177Lu]-TPP can be an interesting tumor targeting agent due to low liver uptake and very low absorbed dose of approximately 0.036 to the total body of human.

Radiopharmaceuticals/internal dosimetry; Lutetium-177; 177Lu-tetraphenyl porphyrin/biodistribution; Porphyrins/biodistribution; Medical internal radiation dose; MIRD


Devido às propriedades interessantes do 177Lu e da avidez tumoral das tetrafenil porfirinas (TPP), desenvolveu-se a 177Lu-tetrafenil porfirina como composto terapêutico potencial. 177Lu de atividade específica de 2,6-3 GBq/mg foi obtido por irradiação de amostra de Lu2O3 com fluxo térmico de nêutrons de 4 × 1013 n.cm-2.s-1 . Sintetizou-se a tetrafenil porfirina e marcou-se com 177Lu. A pureza radioquímica do complexo foi estudada usando método de Cromatografia Instantânea de Camada Delgada ( ITLC). A estabilidade do complexo foi checada na formulação final e no ser humano por 48 h. A biodistribuição do composto marcado em órgãos vitais de ratos do tipo selvagem foi estudada por mais de 7 dias. A dose absorvida para cada órgão humano foi calculada pelo método da Dose Médica de Radiação Interna (MIRD). Estudo farmacocinético comparativo detalhado foi efetuado para o cátion 177Lu e para o [177Lu]-TPP. O complexo foi preparado com pureza radioquímica >97±1% e atividade específica de 970-1000 MBq/mmol. Os dados de biodistribuição e os resultados dosimétricos mostraram que todos os tecidos receberam uma dose absorvida aproximadamente insignificante devido à rápida excreção do complexo pelo trato urinário. O [177Lu]-TPP pode ser um agente interessante de direcionamento do tumor devido à baixa captação pelo fígado e pela dose bem baixa absorvida, de, aproximadamente, 0,036 do corpo humano total.

Radiofármacos/dosimetria interna; Lutécio-177; 177Lu-tetrafenil porfirina/biodistribuição; Porfirinas/biodistribuição; Dose Médica de Radiação Interna


INTRODUCTION

Nowadays, radiopharmaceuticals are widely used for diagnostic and therapeutic purposes. An ideal radiopharmaceutical should lead to substantially greater accumulation rate in the target organ while the accumulation in other organ should be as low as possible. Therefore, scientists have paid attention to new ligands to differentiate between malignant and normal cells (Sanderson et al., 1972SANDERSON, D.R.; FONTANA, R.S.; LIPSON, R.L.; BALDES, EJ. Hematoporphyrin as a diagnostic tool. Cancer, v.30, p.1368-1372, 1972.).

Porphyrin is a heterocyclic macrocycle derived from four pyrrole-like subunits that plays an important role in biological transfer systems. Various porphyrin complexes have shown interesting tumor-avid activity in vitro and in vivo (Subbarayan et al., 2001SUBBARAYAN, M.; SHETTY, S.J.; SRIVASTAVA, T.S.; NORONHA, O.P.D.; SAMUEL, A.M.; MUKHTAR, H. Water-soluble 99mTc-labeled dendritic novel porphyrins tumor imaging and diagnosis. Biochem. Biophys. Res. Commun., v.281, p.32-36, 2001.; Das et al., 2008DAS, T.; CHAKRABORTY, S.; SARMA, H.D.; BANERJEE, S. A novel ]109Pd] palladium labeled porphyrin for possible use in targeted radiotherapy. Radiochim. Acta., v.96, p.427-433, 2008.; Bonnett, 1995BONNETT, R. Photosensitisers of porphyrin and phthalo-cyanine series for photodynamic therapy. Chem. Rev., v.24, p.19-33, 1995.; Jori, 1996JORI, G. Tumour photosensitizers: approaches to enhance selectivity and efficiency of photodynamic therapy. J. Photochem. Photobiol. B: Biol., v.36, p.87-93, 1996.).

Radiolabeled porphyrins have been developed for the therapeutic purposes such as, 109Pd-protoporphyrins (Fawwaz et al., 1974FAWWAZ, R.A.; FRYE, F.; LOUGHMAN, W.D.; HEMPHILL, W. Survival of skin homografts in dogs injected with 109Pd-protoporphyrin. J. Nucl. Med., v.15, p.997-1002, 1974.), 109Pd-porphyrins (Fawwaz, Hemphill, Winchell, 1971FAWWAZ, R.A.; HEMPHILL, W.; WINCHELL, HS. Potential use of 109Pd-porphyrin complexes for selective lymphatic ablation. J. Nucl. Med., v.12, p.231-236, 1971.), 109Pd-derivitized porphyrins (Chakraborty et al., 2007CHAKRABORTY, S.; DAS, T.; BANERJEE, S.; SARMA, H.D.; VENKATESH, M. Preparation and preliminary biological evaluation of a novel 109Pd labeled porphyrin derivative for possible use in targeted tumor therapy. Q. J. Nucl. Med. Mol. Imaging., v.15, p.16-23, 2007.), 188Re-porphyrins (Jia, Deng, Pu, 2007JIA, Z.; DENG, H.; PU, M. Synthesis and preliminary biological studies of the novel conjugate 188Re-labeledmeso-tetrakis(4-sulfophenyl)porphyrin in mice. Nucl. Med. Bio., v.34, p.643-649, 2007.; Sarma et al., 2010SARMA, H.D.; DAS, T.; BANERJEE, S.; VENKATESH, M.; VIDYASAGAR, P.B.; MISHRA, KP. Biologic Evaluation of a Novel 188Re-Labeled Porphyrin in Mice Tumor Model. Cancer Biother. Radiopharm., v.25, p.47-54, 2010.), 123I-Porphyrins (Jae Hak et al., 2007JAE HAK, L.; BYUNG SEOK, M.; TAE SUP, L.; DAE YOON, C.; KWON SOO, C.; GI JEONG, C. Synthesis and biologic evaluation of I-123-labeled porphyrin derivative as a potential tumor-imaging agent. Cancer. Biother. Radiopharm., v.22, p.853-862, 2007.). Various radiolabeled porphyrin complexes such as 57Co-porphyrins (Hambright et al., 1976HAMBRIGHT, P.; SMART, J.C.; McRAE J.; NOHR, M.L.; YANO, Y.; CHU, P.; BEARDEN, AJ. Tumor imaging with 57cobalt(III)-sandwich complexes and 57cobalt(III)-porphyrins. Inorg. Nucl. Chem. Letters., v.12, p.217-222, 1976.), 99mTc-porphyrin (Murugesan et al., 2001MURUGESAN, S.; SHETTYC, S.J.; SRIVASTAVA, T.S.; NORONHA, O.P.D.; SAMUEL, AM. A technetium-99m-labeled cyclam acid porphyrin (CAP) for tumour imaging. Applied. Radiat. Isotopes, v.55, p.641-646, 2001.; Wang, Lin, Lin, 2010WANG, A.Y.; LIN, J.L.; LIN, WC. Studies on the porphine labeled with 99mTc-pertechnetate. J. Radioanal. Nucl. Chem., v.284, p.121-128, 2010.), and 111In-porphyrin (Fazaeli et al., 2012FAZAELI, Y.; JALILIAN, A.R.; AMINI, M.M.; ABOUDZADEH-ROVAIS, M.R.; SHAFAEE, K.; MIRZAI, M.; RAHIMINEJAD, A. Radiosynthesis and biological evaluation of [111In]-5,10,15,20-tetrakis(pentafluorophenyl) porphyrin complex as a possible imaging agent. IJNESE, v.2, p.28-32, 2012.) have also been introduced for imaging.

While the masive accumulation of 109Pd-porphyrins was indicated in fibrosarcoma tumours (Chakraborty et al., 2007CHAKRABORTY, S.; DAS, T.; BANERJEE, S.; SARMA, H.D.; VENKATESH, M. Preparation and preliminary biological evaluation of a novel 109Pd labeled porphyrin derivative for possible use in targeted tumor therapy. Q. J. Nucl. Med. Mol. Imaging., v.15, p.16-23, 2007.), I-123-labeled porphyrin demonstrated high focal accumulation in the B16-F10 melanoma tumor (Jae Hak et al., 2007JAE HAK, L.; BYUNG SEOK, M.; TAE SUP, L.; DAE YOON, C.; KWON SOO, C.; GI JEONG, C. Synthesis and biologic evaluation of I-123-labeled porphyrin derivative as a potential tumor-imaging agent. Cancer. Biother. Radiopharm., v.22, p.853-862, 2007.). (188)Re-labeled 5,10,15,20-tetrakis[3,4-bis(carboxymethyleneoxy)phenyl]porphyrin has also shown specific affinity toward the fibrosarcoma and thymic lymphoma tumors in mice (Sarma et al., 2010SARMA, H.D.; DAS, T.; BANERJEE, S.; VENKATESH, M.; VIDYASAGAR, P.B.; MISHRA, KP. Biologic Evaluation of a Novel 188Re-Labeled Porphyrin in Mice Tumor Model. Cancer Biother. Radiopharm., v.25, p.47-54, 2010.).

Accumulation of the radiolabeled prophyrins in tumour is dependent on various parameters such as porphyrin structure, choice of radioisotope, pH, the presence of inflammation and many other factors. However, the balance between hydrophilicity and lipophilicity is also recognized as an important factor in tumour accumulation. Whereas, lipophilicity of the agent plays an important role in tumor accumulation, hydrophilicity is a significant key in the clearance of the agent from the non-target organs. Therefore, a balance between these two properties is necessary for developing a suitable agent and for contributing to the challenge in designing suitable derivatives of porphyrin (Das et al., 2010DAS, T.; CHAKRABORTY, S.; SARMA, H.D.; BANERJEE, S.; VENAKATESH, M. A novel 177Lu-labeled porphyrin for possible use in targeted tumor therapy. Nucl. Med. Bio., v.37, p.655-663, 2010.).

177Lu decays with a half life of 6.73 d by emission β-particles with maximum energy of 497 keV (78.6%) and γ-photons of 112 keV (6.4%) and 208 keV (11%) to stable 177Hf (TOI, 1993). Due to these good physical characteristics as well as the feasibility of large-scale production in adequate specific activity and radionuclidic purity using a moderate flux reactor, 177Lu has been considered as a promising radionuclide for developing therapeutic radiopharmaceuticals due to its suitable half-life.

177Lu-radiopharmaceuticals have been developed and used in the therapy of various diseases and malignancies, such as somatostatin receptor radiotherapy (Bodei et al., 2009BODEI, L.; FERONE, D.; GRANA, C.M.; CREMONESI, M.; SIGNORE, A.; DIERCKX, R.A.; PAGANELLI, G. Peptide receptor therapies in neuroendocrine tumors. J. Endocrinol. Invest., v.32, p.360-369, 2009.), radioimmunotherapy (Michel et al., 2005MICHEL, R.B.; ANDREWS, P.M.; ROSARIO, A.V.; GOLDENBERG, D.M.; MATTES, MJ. 177Lu-antibody conjugates for single-cell kill of B-lymphoma cells in vitro and for therapy of micrometastases in vivo. Nucl. Med. Biol., v.32, p.269-278, 2005.), bone palliation therapy (Chakraborty et al., 2008aCHAKRABORTY, S.; DAS, T.; BANERJEE, S.; BALOGH, L.; CHAUDHARI, P.R.; SARMA, H.D.; POLYÁK, A.; MÁTHÉ, D.; VENKATESH, M.; JANOKI, G.; PILLAI, MR. 177Lu-EDTMP: a viable bone pain palliative in skeletal metastasis. Cancer Biother. Radiopharm., v.23, p.202-213, 2008a.) and radiosynovectomy (Chakraborty et al., 2006CHAKRABORTY, S.; DAS, T.; BANERJEE, S.; SARMA, H.D.; VENKATESH, M. Preparation and preliminary biological evaluation of 177Lu-labeled hydroxyapatite as a promising agent for radiation synovectomy of small joints. Nucl. Med. Commun., v.27, p.661-668, 2006.; Chakraborty et al., 2008bCHAKRABORTY, S.; DAS, T.; SARMA, H.D.; VENKATESH, M.; BANERJEE, S. Preparation and preliminary studies on 177Lu-labeled hydroxyapatite particles for possible use in the therapy of liver cancer. Nucl. Med. Biol., v.35, p.589-597, 2008b.). 177Lu-5,10,15,20-tetrakis[4-carboxymethyleneoxyphenyl] porphyrin have recently been developed and have shown active tumor uptake in mice bearing fibrosarcoma tumors (Das et al., 2010DAS, T.; CHAKRABORTY, S.; SARMA, H.D.; BANERJEE, S.; VENAKATESH, M. A novel 177Lu-labeled porphyrin for possible use in targeted tumor therapy. Nucl. Med. Bio., v.37, p.655-663, 2010.).

According to the interesting pharmacological properties of porphyrins such as solubility in serum, rapid wash-out, tumor avidity and feasible complexation with various bi/tri-valent metals (Falk, 1975FALK, JE. Porohyrins and metalloporphyrins. New York: Elsevier Science Publishing, 1975.), the idea of developing a possible tumor targeting agent by incorporating 177Lu into a suitable porphyrin ligand, i.e. TPPH2 was investigated (Figure 1).

Figure 1.
Structure of TPPH2.

As for the amount of energy uptake in any organs by ionizing radiation, the absorbed dose, plays an important role in evaluating the risks associated with the administration of radiopharmaceuticals and thus the maximum amount of activity that should be undertaken (Stabin et al., 1999STABIN, M.G.; TAGESSON, M.; THOMAS, S.R.; LJUNGBERG, M.; STRAND, SE. Radiation dosimetry in nuclear medicine. Appl. Radiat. Isot., v.50, p.73-87, 1999.). In nuclear medicine, the most commonly used method for calculation of the internal dose estimates is the one developed by the medical internal radiation dose (MIRD) committee (Stabin, 1996STABIN, MG. MIRDOSE: personal computer software for internal dose assessment in nuclear medicine. J. Nucl. Med., v.37, p.538-546, 1996.) summarized in MIRD primer (Loevinger, Budinger, Watson, 1988LOEVINGER, R.; BUDINGER, T.; WATSON, E. MIRD primer for absorbed dose calculations. New York: Society of Nuclear Medicine, 1988. 128 p.).

In this work, we endeavour to report, synthesis, radiolabeling, quality control and biodistribution studies of 177Lu-TPP in wild-type rats. The time/decay diagrams for the labeled compound in vital organs were plotted compared to lutetium cation. Also the partition coefficient of the complex was calculated and the absorbed dose to each organ of human was evaluated by biodistribution studies in rats by MIRD method.

MATERIAL AND METHODS

177Lu was produced at Tehran Research Reactor. Chemicals were purchased from the Aldrich Chemical Co. (Gemany). NMR spectra were obtained on a FT-80 Varian instrument (80 MHz) with tetramethylsilane as an internal standard. Infrared spectrum was measured on a Perkin-Elmer 781 spectrometer by means of a KBr disc. Mass spectrum was recorded by a Finnigan Mat TSQ-70 Spectrometer. Thin layer chromatography (TLC) for cold compounds was performed on polymer-backed silica gel (F 1500/LS 254, 20 × 20 cm, TLC Ready Foil, Schleicher & Schuell(r), Germany). Normal saline and sodium acetate used for labeling were of high purity and had been filtered through 0.22 μm Cativex filters. Instant thin layer chromatography (ITLC) was performed by counting Whatman No. 2 papers using a thin layer chromatography scanner, Bioscan AR2000, Bioscan Europe Ltd. (France). Biodistribution data were obtained by counting normal saline washed tissues after weighing on a Canberra TM high purity germanium (HPGe) detector (model GC1020-7500SL). Radionuclidic purity was checked with the same detector. For activity measurement of the samples a CRC Capintech Radiometer (NJ, USA) was utilized. All calculations as well as tissue count were based on the 112 keV peak of 177Lu. Animal studies were performed in accordance with the United Kingdom Biological Council's Guidelines on the Use of Living Animals in Scientific Investigations, 2nd ed.

Production and quality control of 177LuCl3 solution

177Lu was produced by irradiation of natural Lu2O3target (1 mg) at a thermal neutron flux of approximately 4 × 1013n/cm2.s for 5 days at Tehran Research Reactor (TRR) according to the reported procedures (Yousefnia et al., 2011YOUSEFNIA, H.; RADFAR, E.; JALILIAN, A.R.; BAHRAMI-SAMANI, A.; SHIRVANI-ARANI, S.; ARBABI, A.; GHANNADI-MARAGHEH, M. Development of 177Lu-DOTA-anti-CD20 for radioimmunotherapy. J. Radioanal. Nucl. Chem., v.287, p.199-209, 2011.). The irradiated target was dissolved in 200 µL of 1.0 M HCl, to prepare 177LuCl3 and diluted to the appropriate volume with ultra pure water, to produce a stock solution of final volume of 5 mL with approximately 2.8 GBq. The mixture was filtered through a 0.22 µm biological filter and sent for use in the radiolableing stage in the process. For radionuclidic purity determination, the sample was checked by gamma-ray spectroscopy on an HPGe detector for 5 h based on two major photons of 177Lu (6.4% of 0.112 MeV and 11% of 0.208 MeV). The radiochemical purity of the 177LuCl3was checked using 2 solvent systems for ITLC (A: 10 mM DTPA pH.4 and B: ammonium acetate 10%:methanol (1:1)).

Preparation of Tetraphenyl Porphyrin (TPPH2)

This compound was prepared according to the reported method using freshly distilled benzaldehyde, pyrrole and propionic acid followed by oxidation (Adler et al., 1967). Yield; 20%, m.p.> 248-250 ºC. 1H NMR (CDCl3) δ (ppm) -2.8 (2 H, NH), 7.71-7.82 (12 H), 8.14-8.27 (8 H), 8.85 (8 H). 13C-NMR (CDCl3) δ (ppm) 120.20 (C), 126.74 (CH), 127.76 (CH), 131.16 (CH), 134.62 (CH), 142.22 (C), 145.6 (C).UV (toluene) λmax (ε) = 418 nm (413200), 514 (19060), 549 (8080), 594 (5380), 648 (3870). IR (KBr) 3320, 3055, 3025, 1595.

Preparation of [177Lu]-TPP

0.2 ml of 177LuCl3 with 111 MBq radioactivity was transferred to a 5 mL-borosilicate vial and heated to dryness by using a flow of N2gas at 50-60 ºC, followed by the addition of fifty microliters of TPP in absolute ethanol (1 mg/mL ≈ 81 nmoles) and 450 microliters of acetate buffer pH 5 (0.1 M). The mixture vortexed at 25 ºC for 4 h. The final solution was then passed through a 0.22 μm filter and the radiochemical purity was checked by ITLC. For this purpose, 5 μL of the final solution was spotted on a chromatography Whatman No. 2 paper, and developed in two mobile phase mixtures, A: water:acetonitrile (3:1) and B: water:acetonitrile (1:3).

Determination of partition coefficient

Partition coefficient (log P) of [177Lu]-TPP was calculated. A mixture of 1 mLof 1-octanol and 1 mL of isotonic acetate-buffered saline (pH 7) containing approximately 3.7 MBq of the radiolabeled complex at 37 °C was vortexed 1 min and left 5 min. Following centrifugation at >1200 g for 5 min, the octanol and aqueous phases were sampled and counted in an automatic well-type counter. A 500 μL sample of the octanol phase from this experiment was shaken again thrice with fresh buffer samples. The reported log P values are the average of the second and third extractions from three to four independent measurements.

Stability tests

The stability of the complex was checked according to the conventional ITLC method. A sample of [177Lu]-TPP (37 MBq) was kept at room temperature for 2 days while being checked by ITLC at time intervals in order to check stability in final product using above chromatography system. For serum stability studies, to 36.1 MBq of [177Lu]-TPP was added 500 μL of freshly collected human serum and the resulting mixture was incubated at 37 °C for 5 h; Aliquots (5 μL) were analyzed by ITLC.

Biodistribution in wild-type rats

The distribution of 177Lucl3 and the radiolabeled complex among tissues were determined for wild-type rats. 50-100 µL of 177Lu-TPP or 177LuCl3 solutions with 1.85 MBq radioactivity were injected intravenously via their tail veins. The total amount of radioactivity injected into each rat was measured by counting the 1 mL syringe before and after injection in a dose calibrator with fixed geometry. The animals were sacrificed by CO2asphyxiation at selected times after injection (2, 4, 24, 48, 120 and 168 h). The tissues (blood, heart, lung, brain, intestine, feces, skin, stomach, kidneys, liver, muscle and bone) were weighed and rinsed with normal saline and their specific activities were determined with an HPGe detector equipped with a sample holder device as percent of injected dose per gram of tissues.

Dosimetric studies

The absorbed dose of each human organ was calculated by MIRD method based on biodistribution data in wild-type rats. The accumulated activity in animals was extrapolated to the accumulated activity in humans by the proposed method of Sparks et al. (eq. 1) (Sparks, Aydogan, 1996SPARKS, R.B.; AYDOGAN, B. Comparison of the effectiveness of some common animal data scaling techniques in estimating human radiation dose Sixth International Radiopharmaceutical Dosimetry Symposium. Oak Ridge, TN: Oak Ridge Associated Universities, 1996. p.705-716.).

where à is the accumulated activity in the source organs and can be calculated by the equation 2.

It should be noticed that A (t) is the activity of each organ at time t.

The accumulated source activity for each organ of animals was calculated by plotting the percentage-injected dose versus time for each organ and computing the area under the curves. For this purpose, the data points which represent the percentage-injected dose were created. The curves were extrapolated to infinity by fitting the tail of each curve to a monoexponential curve with the exponential coefficient equal to physical decay constant of 177Lu. Then the area under the curve was calculated. In order to extrapolate this accumulated activity to human, the mean weights of each organ for standard human were used (Table I).

Table I.
The mean weights of organs for human with standard weight (ICRP 89, 2001)

The radiation absorbed dose was calculated by MIRD formulation (Henrichs, Kaul, Roedler, 1982HENRICHS, K.; KAUL, A.; ROEDLER, HD. Estimation of age-dependent internal dose from radiopharmaceuticals. Phys. Med. Biol., v.27, p.775-784, 1982.):

where D(rk) is the absorbed dose of the target organ, and S(rk ← rh) called S factor which is defined as the mean absorbed dose to the target region rk per unit accumulated activity in the source region rh. S factor represents the physical decay characteristics of the radionuclide, the range of the emitted radiations, and the organ size and configuration (Bevelacqua, 2005BEVELACQUA, JJ. Internal dosimetry primer. Radiat. Prot. Manage., v.22, p.7-17, 2005.) expressed in mGy/MBq.s. The S factors have been taken from the OLINDA software (OLINDA, 2007OLINDA - Organ Level Internal Dose Assessment Code (Version 1.1), copyright Vanderbilt University, (2007).).

RESULTS AND DISCUSSION

Radionuclide production

The radionuclide was prepared in a research reactor according to the regular methods with a range of specific activity 2.6-3 GBq/mg for radiolabeling use. The obtained radionuclidic purity was 99.98% (Figure 2). Furthermore, half-life of the 177Lu was also studied by counting the sample at different time intervals. The decay scheme for the radionuclide is shown in Figure 3.

Figure 2.
Gamma-ray spectrum for 177LuCl3 solution used in this study.

Figure 3.
Decay scheme for 177Lu used in this study.

The radioisotope was dissolved in acidic media as a starting sample and was further diluted and evaporated for obtaining the desired pH and volume followed by sterile filtering. The radiochemical purity of the 177Lu solution was checked in two solvent systems: in 10 mM DTPA, free Lu3+ cation as a complex in more lipophilic LuDTPA form migrates to higher Rf, while small radioactive fraction remains in its origin which could be related to other Lu ionic species, not forming LuDTPA complex, such as LuCl4 -, etc. and/or colloids.

On the other hand, 10% ammonium acetate:methanol mixture was also used for the determination of radiochemical purity. In this solvent system, the fast eluting species were possibly Lu-177 cations, other than Lu3+ and the remaining fraction at Rf.0 was a possible mixture of Lu3+ and/or colloids. The difference in values of impurity in two solvent systems is possibly due to the presence of colloidal impurity in the sample (Figure 4).

Figure 4.
ITLC chromatograms of 177LuCl3 solution in DTPA solution (pH. 4) (left) and 10% ammonium acetate:methanol (1:1) solution (right) using Whatman No. 2.

Preparation of [177Lu]- TPP

The synthetic scheme for radiolabeling of TPP with 177LuCl3 is demonstrated in Figure 5. Because of the engagement of NH polar functional groups in its structure, labeling of TPPH2 with lutetium cation affects its chromatographic properties and the final complex is more lipophilic. Two different chromatographic systems were used. Using water/acetonitrile (1:3) mixture, free lutetium remains in its origin of the paper as a single peak, while the radiolabeled compound migrates to higher Rf. Using a more polar mobile phase, acetonitrile:water (1:3), free lutetium cation migrated to a higher Rf, while the radiolabeled compound remained at the origin (Figure 6).

Figure 5.
Synthetic scheme for radiolabeling of TPP with 177LuCl3.

Figure 6.
ITLC chromatograms of 177LuCl3 and 177Lu-TTP on Whatman No. 2 paper using water:acetonitrile (1:3) [up (a & b)] and acetonitrile:water (1:3) [down (c & d)] mixtures.

Partition coefficient

As expected, the lipophilicity of the [177Lu]-TPP compound is rather high. The measured octanol/water partition coefficient, P, for the complex was found to depend on the pH of the solution. At the pH 7, the log P was 1.63.

Stability

The stability of [177Lu]-TPP prepared complex at room temperature was checked up to 48 hours. The radiochemical purity of the complex remained at 98% for 2 days. Also the stability of the complex was determined at 37 °C for 48 h and the data were almost consistent with the final solution stability.

Biodistribution studies of 177Lucl3 and 177Lu-TPP in wild-type rats

The animals were sacrificed by CO2 asphyxiation at selected times after injection (2, 4, 24, 48 and 168 h). The biodistribution data show that the liver uptake of the cation is comparable to many other radio-metals mimicking ferric cation accumulation; about 3% of the activity accumulates in the liver after 48 h. Binding of 177Lu by transferrin and transport to the liver appears to be the route of accumulation (Figure 7).

Figure 7.
Percentage of injected dose per gram (ID/g%) of 177LuCl3 in wild-type rat tissues at 2, 4, 24, 48 and 168 h post injection (ID/g%: percentage of the injected dose per gram of tissue calculated based on the area under curve of 112 keV peak in gamma spectrum) (n=5).

As it can be seen from Figure 5, the blood content is low at all time intervals, which shows the rapid removal of activity in the circulation. The lung, muscle and also skin do not demonstrate significant uptake while it is in accordance with other cations accumulation. A 5% bone uptake is observed for the cation at 168 h. The spleen also has uptake (1%) possibly related to reticuloendothelial uptake. The kidney plays an important role in 177Lu cation excretion especially after 24 h (1%). Biodistribution of 177Lu-TPP in different organs of wild-type rats is shown in Figure 8.

Figure 8.
Biodistribution of [177Lu]-TPP (1.85 MBq) in wild type rats 2, 24, 120 and 168 h after iv injection via tail vein (ID/g%: percentage of injected dose per gram of tissue calculated based on the area under curve of 112 keV peak in gamma spectrum) (n=5)

Comparison of vital organs uptake for 177Lu-TPP and 177LuCl3 demonstrates kinetic pattern difference for both species. 177Lu cation is accumulated in the liver within the first 24 h post injection slightly, while 177Lu-TPP second major excretion route is through the liver and slow uptake of less than 1% is observed in 168 h for the radiolabeled compound (Figure 9).

Figure 9.
Comparative organ uptake of 177LuCl3 and 177Lu- TPP in wild-type rats.

Intestinal activity increases in 177Lu3+ after 7 days as a consequence of the liver excretion through GI tract, however, in case of Lu porphyrin, the amount of activity decreases due to the low liver uptake compared with urinary excretion. As shown earlier, 177Lu cation is slightly absorbed in the skeletal system (5%) while the labeled compound almost shows no uptake in the bone.

Since the urinary tract is a major route of excretion of the porphyrins, the amount of the kidney activity is maximum for the labeled compound especially after 24 h, however in 7days major urinary excretion is observed in free cation. The circulation wash-out for the labeled compound is observed while a lesser amount of activity in blood is observed for free Lu cation.

Dosimetric studies

Dosimetric evaluation in human organs was made by MIRD method based on biodistribution in rat organs. The clearance curves from each organ of the rats are shown in Figure 10. The absorbed dose in each organ of human after injection of 177Lu-TPP is given in Table II.

Table II.
Absorbed dose in each organ of human after injection of 177Lu-TPP

Figure 10.
The clearance curves from each organ of the rats.

CONCLUSION

Total labeling and formulation of [177Lu]-TPP took about 4 h (radiochemical purity: >97 ± 1% ITLC, specific activity, 970-1000 MBq/mmol). The complex was stable in final formulation and human serum at least for 24 h. The biodistribution of the labeled compound in vital organs of wild-type rats was studied up to 7 days post injection. The accumulation of the tracer in other tumor models is under investigation due to the low liver uptake and rapid excertion through the urinary tract. Biodistribution data and dosimetric results showed that all tissues receive virtually insignificant absorbed dose due to rapid excretion of the complex through the urinary tract. [177Lu]-TPP can be an interesting tumor targeting agent due to the low liver uptake and very low absorbed dose of approximately 0.036 mSv/MBq to the total body of human.

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Publication Dates

  • Publication in this collection
    Apr-Jun 2015

History

  • Received
    26 Feb 2014
  • Accepted
    04 Aug 2014
Universidade de São Paulo, Faculdade de Ciências Farmacêuticas Av. Prof. Lineu Prestes, n. 580, 05508-000 S. Paulo/SP Brasil, Tel.: (55 11) 3091-3824 - São Paulo - SP - Brazil
E-mail: bjps@usp.br