Validation of analytical HPLC with post-column injection as a method for rapid and precise quantification of radiochemical yields

Accurate assessment of isolated radiochemical yields (RCYs) is a prerequisite for efficient and reliable optimization of labeling reactions. In practice, radiochemical conversions (RCCs) determined by HPLC analysis of crude reaction mixtures are often used to estimate RCYs. However, incomplete recovery of radioactivity from the stationary phase can lead to significant inaccuracies if RCCs are calculated based on the activity eluted from the column (i.e. the summed integrals of all peaks). Here, we validate a simple and practical method that overcomes problems associated with retention of activity on the column by determination of the total activity in the sample using post-column injection. Post-column injections were carried out using an additional injection valve, which was placed between the outlet of the HPLC column and the inlet of the detectors. 2-[18F]Fluoropyridine ([18F]FPy) and 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CPFPX) were prepared with radiochemical purities of > 99.8% and mixed with [18F]fluoride at a ratio of 1:1 to simulate reaction mixtures obtained by radiolabeling reactions with an RCC of 50%. The samples were analyzed on three different C18 HPLC columns using neutral and acidic mobile phases. RCCs determined using the summed area of all peaks in the chromatograms were compared with those determined using post-column injection. Additionally, RCCs determined by post-column injection were corrected for activity losses before, during and after radiosyntheses to afford analytical RCYs, which were compared with isolated RCYs. Determination of RCCs based on the summed area of all peaks gave correct results under certain chromatographic conditions, but led to overestimation of the actual RCCs by up to 50% in other cases. In contrast, determination of RCCs using post-column injection provided precise results in all cases, and often significantly reduced analysis time. Moreover, analytical RCYs calculated from RCCs determined by post-column injection showed excellent agreement with isolated RCYs (<3% deviation). In conclusion, HPLC analysis using post-column injection enables reliable determination of RCCs independent of the chromatographic conditions and, together with a simple activity balance, rapid and accurate prediction of isolated RCYs.

Radiolysis-Associated Decrease in Radiochemical Purity of 177Lu-Radiopharmaceuticals and Comparison of the Effectiveness of Selected Quenchers against This Process

The radiolytic degradation of vector molecules is a major factor affecting the shelf life of therapeutic radiopharmaceuticals. The development of time-stable dosage forms of radiopharmaceuticals is the key to their successful implementation in clinical practice. Using [¹⁷⁷Lu]Lu-PSMA-617 molecule as an example, the time dependence of the change in radiochemical purity (RCP, %) under radiolysis conditions was studied. The dependence of [¹⁷⁷Lu]Lu-PSMA-617 radiolysis on parameters such as time, radionuclide activity, buffer agent concentration, precursor amount, and preparation volume was evaluated. It was shown that the absorbed dose was the dominant factor influencing the RCP. The RCP value is inversely proportional to the absorbed dose in the [¹⁷⁷Lu]Lu-PSMA-617 preparation and has an exponential dependence. The lutetium-177 dose factor ψ (Gy·mL·MBq^-1) and PSMA-617 concentration-dependent dose constant κ (Gy^-1) were evaluated for absorbed dose estimation via computer modeling, chemical dosimetry, and radiochemical purity monitoring under various conditions. The further refinement and application of the dependencies found can be useful for predicting the RCP value at the stage of optimizing the composition of the finished dosage form of therapeutic radiopharmaceuticals. The influence of the buffer agent (sodium acetate) concentration on [¹⁷⁷Lu]Lu-PSMA-617 radiolytic degradation was shown and should be considered both when developing a dosage form, and when comparing the results of independent studies. The effectiveness of the addition of various stabilizing agents, such as DMSA, cysteine, gentisic acid, vanillin, methionine, adenine, dobesilic acid, thymine, uracil, nicotinamide, meglumine, and mannitol, in suppressing the effects of radiolysis was evaluated.

Synthesis, Complexation Properties, and Evaluation of New Aminodiphosphonic Acids as Vector Molecules for 68Ga Radiopharmaceuticals

Two new aminodiphosphonic acids derived from salicylic acid and its phosphonic analogue were prepared through a simple and efficient synthesis. 2-[(2-Amino-2,2-diphosphono)ethyloxy]-benzoic acid 8 and 2-[(2-amino-2,2-diphosphono)ethyloxy]-5-ethyl-phenylphosphonic acid 9 were evaluated for their applicability as ⁶⁸Ga binding bone-seeking agents. Protonation constants of 8 and 9 and stability constants of the Ga³⁺ complexes with 8 and 9 in water were determined. The stability constant of Ga³⁺ complex with fully phosphorylated acid 9 (logK_GaL = 31.92 ± 0.32) significantly exceeds stability constant of Ga³⁺ complex with 8 (logK_GaL = 26.63 ± 0.24). Ligands 8 and 9 are as effective for Ga³⁺ cation binding as ethylenediamine-N,N’-diacetic-N,N’-bis(methy1enephosphonic) acid and ethylenediamine-N,N,N’,N’-tetrakis(methylenephosphonic) acid, respectively. The labelling process and stability of [⁶⁸Ga]Ga-8 and [⁶⁸Ga]Ga-9 were studied. Both 8 and 9 readily form ⁶⁸Ga-complexes stable to ten-fold dilution with saline. However, in fetal bovine serum, only [⁶⁸Ga]Ga-9 was stable enough to be subject to biological evaluation. It was injected into rats with bone pathology and aseptic inflammation of soft tissues. For [⁶⁸Ga]Ga-9 in animals with a bone pathology model in 60 and 120 min after injection, a slight accumulation in the pathology site, stable blood percentage level, and moderate accumulation in the liver were observed. For animals with an aseptic inflammation, the accumulation of [⁶⁸Ga]Ga-9 in the pathology site was higher than that in animals with bone pathology. Moreover, the accumulation of [⁶⁸Ga]Ga-9 in inflammation sites was more stable than that for [⁶⁸Ga]Ga-citrate. The percentage of [⁶⁸Ga]Ga-9 in the blood decreased from 3.1% ID/g (60 min) to 1.5% ID/g (120 min). Accumulation in the liver was comparable to that obtained for [⁶⁸Ga]Ga-citrate.

Pharmacokinetic Properties of 68Ga-labelled Folic Acid Conjugates: Improvement Using HEHE Tag

The folate receptor (FR) is a promising cell membrane-associated target for molecular imaging and radionuclide therapy of cancer (FR-α) and potentially also inflammatory diseases (FR-β) through use of folic acid-based radioconjugate. FR is often overexpressed by cells of epithelial tumors, including tumors of ovary, cervix, endometrium, lungs, kidneys, etc. In healthy tissues, FR can be found in small numbers by the epithelial cells, mainly in the kidneys. Extremely high undesired accumulation of the folate radioconjugates in the renal tissue is a main drawback of FR-targeting concept. In the course of this work, we aimed to reduce the undesirable accumulation of folate radioconjugates in the kidneys by introducing a histidine/glutamic acid tag into their structure. Two folic acid based compounds were synthesized: NODAGA-1,4-butanediamine-folic acid (FA-I, as control) and NODAGA-[Lys-(HE)₂]-folic acid (FA-II) which contains a (His-Glu)₂ fragment. In vitro studies with FR (+) cells (KB and others) showed that both compounds have specificity for FR. Introduction of (HE)₂-tag does not affect FR binding ability of the conjugates. In vivo biodistribution studies with normal laboratory animals, as well as with KB tumor bearing animals, were carried out. The results showed that introduction of the (HE)₂ tag into the structure of folate radioconjugates can significantly reduce the accumulation of these compounds in non-target tissues and important organs (the accumulation in the kidneys is reduced 2-4 times), leaving the accumulation in tumor at least at the same level, and even increasing it.

Preparation of Zirconium-89 Solutions for Radiopharmaceutical Purposes: Interrelation Between Formulation, Radiochemical Purity, Stability and Biodistribution

Zirconium-89 is a promising radionuclide for nuclear medicine. The aim of the present work was to find a suitable method for obtaining zirconium-89 solutions for radiopharmaceutical purposes. For this purpose, the ion exchange behavior of zirconium-89 solutions was studied. Radio-TLC (thin layer chromatography) and biodistribution studies were carried out to understand speciation of zirconium-89 complexes and their role in the development of new radiopharmaceuticals. Three methods of zirconium-89 isolation were studied using ZR (hydroxamate) and Chelex-100 resins. It was found that ZR-resin alone is not enough to obtain stable zirconium-89 formulations. An easy and effective method of reconstitution of [⁸⁹Zr]Zr-oxalate to [⁸⁹Zr]Zr-citrate using Chelex-100 resin was developed. Developed procedures allow obtaining [⁸⁹Zr]Zr-oxalate (in 0.1 M sodium oxalate solution) and [⁸⁹Zr]Zr-citrate (in 0.1–1.0 M sodium citrate solution). These solutions are perfectly suitable and convenient for radiopharmaceutical purposes. Our results prove [⁸⁹Zr]Zr-citrate to be advantageous over [⁸⁹Zr]Zr-oxalate. During evaluation of speciation of zirconium-89 complexes, a new TLC method was developed, since it was proved that there is no comprehensive method for analysis or zirconium-89 preparations. The new method provides valuable insights about the content of “active” ionic form of zirconium-89. The interrelation of the chromatographic behavior of zirconium-89 preparations and their biodistribution was studied.