Pharmacokinetic modeling

EANM guidance document: dosimetry for first-in-human studies and early phase clinical trials

The numbers of diagnostic and therapeutic nuclear medicine agents under investigation are rapidly increasing. Both novel emitters and novel carrier molecules require careful selection of measurement procedures. This document provides guidance relevant to dosimetry for first-in human and early phase clinical trials of such novel agents. The guideline includes a short introduction to different emitters and carrier molecules, followed by recommendations on the methods for activity measurement, pharmacokinetic analyses, as well as absorbed dose calculations and uncertainty analyses. The optimal use of preclinical information and studies involving diagnostic analogues is discussed. Good practice reporting is emphasised, and relevant dosimetry parameters and method descriptions to be included are listed. Three examples of first-in-human dosimetry studies, both for diagnostic tracers and radionuclide therapies, are given.

Single-Time-Point Renal Dosimetry Using Nonlinear Mixed-Effects Modeling and Population-Based Model Selection in [177Lu]Lu-PSMA-617 Therapy

The aim of this study was to investigate the accuracy of single-time-point (STP) renal dosimetry imaging using SPECT/CT data, a nonlinear mixed-effects (NLME) model, and a population-based model selection (PBMS) in a large population for 177Lu-labeled prostate-specific membrane antigen therapy. Methods: Biokinetic data (mean ± SD) of [177Lu]Lu-PSMA-617 in kidneys at time points 1 (1.8 ± 0.8 h), 2 (18.7 ± 0.9 h), 3 (42.6 ± 1.0 h), 4 (66.3 ± 0.9 h), and 5 (160.3 ± 24.2 h) after injection were obtained from 63 patients with metastatic castration-resistant prostate cancer using SPECT/CT. Thirteen functions were derived from various parameterizations of 1- to 5-exponential functions. The function's parameters were fitted in the NLME framework to the all-time-point (ATP) data. The PBMS NLME method was performed using the goodness-of-fit test and Akaike weight to select the best function fitting the data. The best function from ATP fitting was used to calculate the reference time-integrated activity and absorbed doses. In STP dosimetry, the parameters of a particular patient with STP data were fitted simultaneously to the STP data at different time points of that patient with ATP data of all other patients. The parameters from STP fitting were used to calculate the STP time-integrated activity and absorbed doses. Relative deviations (RDs) and root-mean-square errors (RMSEs) were used to analyze the accuracy of the calculated STP absorbed dose compared with the reference absorbed dose obtained from the best-fit ATP function. The performance of STP dosimetry using PBMS NLME modeling was compared with the Hänscheid and Madsen methods. Results: The function [Formula: see text] was selected as the best-fit ATP function, with an Akaike weight of 100%. For STP dosimetry, the STP measurement by SPECT/CT at time point 3 (42.6 ± 1.0 h) showed a relatively low mean RD of -4.4% ± 9.4% and median RD of -0.7%. Time point 3 had the lowest RMSE value compared with those at the other 4 time points. The RMSEs of the absorbed dose RDs for time points 1-5 were 23%, 16%, 10%, 20%, and 53%, respectively. The STP dosimetry using the PBMS NLME method outperformed the Hänscheid and Madsen methods for all investigated time points. Conclusion: Our results show that a single measurement of SPECT/CT at 2 d after injection might be used to calculate accurate kidney-absorbed doses using the NLME method and PBMS.

Physiologically based radiopharmacokinetic (PBRPK) modeling to simulate and analyze radiopharmaceutical therapies: studies of non-linearities, multi-bolus injections, and albumin binding

BACKGROUND

We aimed to develop a publicly shared computational physiologically based pharmacokinetic (PBPK) model to reliably simulate and analyze radiopharmaceutical therapies (RPTs), including probing of hot-cold ligand competitions as well as alternative injection scenarios and drug designs, towards optimal therapies.

RESULTS

To handle the complexity of PBPK models (over 150 differential equations), a scalable modeling notation called the "reaction graph" is introduced, enabling easy inclusion of various interactions. We refer to this as physiologically based radiopharmacokinetic (PBRPK) modeling, fine-tuned specifically for radiopharmaceuticals. As three important applications, we used our PBRPK model to (1) study the effect of competition between hot and cold species on delivered doses to tumors and organs at risk. In addition, (2) we evaluated an alternative paradigm of utilizing multi-bolus injections in RPTs instead of prevalent single injections. Finally, (3) we used PBRPK modeling to study the impact of varying albumin-binding affinities by ligands, and the implications for RPTs. We found that competition between labeled and unlabeled ligands can lead to non-linear relations between injected activity and the delivered dose to a particular organ, in the sense that doubling the injected activity does not necessarily result in a doubled dose delivered to a particular organ (a false intuition from external beam radiotherapy). In addition, we observed that fractionating injections can lead to a higher payload of dose delivery to organs, though not a differential dose delivery to the tumor. By contrast, we found out that increased albumin-binding affinities of the injected ligands can lead to such a differential effect in delivering more doses to tumors, and this can be attributed to several factors that PBRPK modeling allows us to probe.

CONCLUSIONS

Advanced computational PBRPK modeling enables simulation and analysis of a variety of intervention and drug design scenarios, towards more optimal delivery of RPTs.

Time-Activity data fitting in molecular Radiotherapy: Methodology and pitfalls

Absorbed radiation doses are essential in assessing the effects, e.g. safety and efficacy, of radiopharmaceutical therapy (RPT). Patient-specific absorbed dose calculations in the target or the organ at risk require multiple inputs. These include the number of disintegrations in the organ, i.e. the time-integrated activities (TIAs) of the organs, as well as other parameters describing the process of radiation energy deposition in the target tissue (i.e. mean energy per disintegration, radiation dose constants, etc). TIAs are then estimated by incorporating the area under the radiopharmaceutical's time-activity curve (TAC), which can be obtained by quantitative measurements of the biokinetics in the patient (typically based on imaging data such as planar scintigraphy, SPECT/CT, PET/CT, or blood and urine samples). The process of TAC determination/calculation for RPT generally depends on the user, e.g., the chosen number and schedule of measured time points, the selection of the fit function, the error model for the data and the fit algorithm. These decisions can strongly affect the final TIA values and thus the accuracy of calculated absorbed doses. Despite the high clinical importance of the TIA values, there is currently no consensus on processing time-activity data or even a clear understanding of the influence of uncertainties and variations in personalised RPT dosimetry related to user-dependent TAC calculation. As a first step towards minimising site-dependent variability in RPT dosimetry, this work provides an overview of quality assurance and uncertainty management considerations of the TIA estimation.

Single-time-point dosimetry using model selection and nonlinear mixed-effects modelling: a proof of concept

PURPOSE

This project aims to develop and evaluate a method for accurately determining time-integrated activities (TIAs) in single-time-point (STP) dosimetry for molecular radiotherapy. It performs a model selection (MS) within the framework of the nonlinear mixed-effects (NLME) model (MS-NLME).

METHODS

Biokinetic data of [¹¹¹In]In-DOTATATE activity in kidneys at T1 = (2.9 ± 0.6) h, T2 = (4.6 ± 0.4) h, T3 = (22.8 ± 1.6) h, T4 = (46.7 ± 1.7) h, and T5 = (70.9 ± 1.0) h post injection were obtained from eight patients using planar imaging. Eleven functions were derived from various parameterisations of mono-, bi-, and tri-exponential functions. The functions' fixed and random effects parameters were fitted simultaneously (in the NLME framework) to the biokinetic data of all patients. The Akaike weights were used to select the fit function most supported by the data. The relative deviations (RD) and the root-mean-square error (RMSE) of the calculated TIAs for the STP dosimetry at T3 = (22.8 ± 1.6) h and T4 = (46.7 ± 1.7) h p.i. were determined for all functions passing the goodness-of-fit test.

RESULTS

The function [Formula: see text] with four adjustable parameters and [Formula: see text] was selected as the function most supported by the data with an Akaike weight of (45 ± 6) %. RD and RMSE values show that the MS-NLME method performs better than functions with three or five adjustable parameters. The RMSEs of TIA_NLME-PBMS and TIA_3-parameters were 7.8% and 10.9% (for STP at T3), and 4.9% and 10.7% (for STP at T4), respectively.

CONCLUSION

An MS-NLME method was developed to determine the best fit function for calculating TIAs in STP dosimetry for a given radiopharmaceutical, organ, and patient population. The proof of concept was demonstrated for biokinetic ¹¹¹In-DOTATATE data, showing that four-parameter functions perform better than three- and five-parameter functions.

A population-based method to determine the time-integrated activity in molecular radiotherapy

BACKGROUND

The calculation of time-integrated activities (TIAs) for tumours and organs is required for dosimetry in molecular radiotherapy. The accuracy of the calculated TIAs is highly dependent on the chosen fit function. Selection of an adequate function is therefore of high importance. However, model (i.e. function) selection works more accurately when more biokinetic data are available than are usually obtained in a single patient. In this retrospective analysis, we therefore developed a method for population-based model selection that can be used for the determination of individual time-integrated activities (TIAs). The method is demonstrated at an example of [¹⁷⁷Lu]Lu-PSMA-I&T kidneys biokinetics. It is based on population fitting and is specifically advantageous for cases with a low number of available biokinetic data per patient.

METHODS

Renal biokinetics of [¹⁷⁷Lu]Lu-PSMA-I&T from thirteen patients with metastatic castration-resistant prostate cancer acquired by planar imaging were used. Twenty exponential functions were derived from various parameterizations of mono- and bi-exponential functions. The parameters of the functions were fitted (with different combinations of shared and individual parameters) to the biokinetic data of all patients. The goodness of fits were assumed as acceptable based on visual inspection of the fitted curves and coefficients of variation CVs < 50%. The Akaike weight (based on the corrected Akaike Information Criterion) was used to select the fit function most supported by the data from the set of functions with acceptable goodness of fit.

RESULTS

The function [Formula: see text] with shared parameter [Formula: see text] was selected as the function most supported by the data with an Akaike weight of 97%. Parameters [Formula: see text] and [Formula: see text] were fitted individually for every patient while parameter [Formula: see text] was fitted as a shared parameter in the population yielding a value of 0.9632 ± 0.0037.

CONCLUSIONS

The presented population-based model selection allows for a higher number of parameters of investigated fit functions which leads to better fits. It also reduces the uncertainty of the obtained Akaike weights and the selected best fit function based on them. The use of the population-determined shared parameter for future patients allows the fitting of more appropriate functions also for patients for whom only a low number of individual data are available.

Important pharmacokinetic parameters for individualization of 177 Lu-PSMA therapy: A global sensitivity analysis for a physiologically-based pharmacokinetic model

PURPOSE

The knowledge of the contribution of anatomical and physiological parameters to interindividual pharmacokinetic differences could potentially be used to improve individualized treatment planning for radionuclide therapy. The aim of this study was therefore to identify the physiologically based pharmacokinetic (PBPK) model parameters that determine the interindividual variability of absorbed doses (ADs) to kidneys and tumor lesions in therapy with ¹⁷⁷ Lu-labeled PSMA-targeting radioligands.

METHODS

A global sensitivity analysis (GSA) with the extended Fourier Amplitude Sensitivity Test (eFAST) algorithm was performed. The whole-body PBPK model for PSMA-targeting radioligand therapy from our previous studies was used in this study. The model parameters of interest (input of the GSA) were the organ receptor densities [R₀ ], the organ blood flows f, and the organ release rates λ. These parameters were systematically sampled NE times according to their distribution in the patient population. The corresponding pharmacokinetics were simulated and the ADs (model output) to kidneys and tumor lesions were collected. The main effect S i and total effect S Ti were calculated using the eFAST algorithm based on the variability of the model output: The main effect S i of input parameter i represents the reduction in variance of the output if the "true" value of parameter i would be known. The total effect S Ti of an input parameter i represents the proportion of variance remaining if the "true" values of all other input parameters except for i are known. The numbers of samples NE were increased up to 8193 to check the stability (i.e., convergence) of the calculated main effects S i and total effects S Ti .

RESULTS

From the simulations, the relative interindividual variability of ADs in the kidneys (coefficient of variation CV = 31%) was lower than that of ADs in the tumors (CV up to 59%). Based on the GSA, the most important parameters that determine the ADs to the kidneys were kidneys flow ( S i  = 0.36, S Ti  = 0.43) and kidneys receptor density ( S i  = 0.25, S Ti  = 0.30). Tumor receptor density was identified as the most important parameter determining the ADs to tumors ( S i and S Ti up to 0.72).

CONCLUSIONS

The results suggest that an accurate measurement of receptor density and flow before therapy could be a promising approach for developing an individualized treatment with ¹⁷⁷ Lu-labeled PSMA-targeting radioligands.

Biodistribution and Multicompartment Pharmacokinetic Analysis of a Targeted α Particle Therapy

Targeted α particle therapy (TAT) is ideal for treating disease while minimizing damage to surrounding nontargeted tissues due to short path length and high linear energy transfer (LET). We developed a TAT for metastatic uveal melanoma, targeting the melanocortin-1 receptor (MC1R), which is expressed in 94% of uveal melanomas. Two versions of the therapy are being investigated: ²²⁵Ac-DOTA-Ahx-MC1RL (²²⁵Ac-Ahx) and ²²⁵Ac-DOTA-di-d-Glu-MC1RL (²²⁵Ac-di-d-Glu). The biodistribution (BD) from each was studied and a multicompartment pharmacokinetic (PK) model was developed to describe drug distribution rates. Two groups of 16 severe combined immunodeficient (SCID) mice bearing high MC1R expressing tumors were intravenously injected with ²²⁵Ac-Ahx or ²²⁵Ac-di-d-Glu. After injection, four groups (n = 4) were euthanized at 24, 96, 144, and 288 h time points for each cohort. Tumors and 13 other organs were harvested at each time point. Isomeric γ spectra were measured in tissue samples using a scintillation γ detector and converted to α activity using factors for γ ray abundance per α decay. Time activity curves were calculated for each organ. A five-compartment PK model was built with the following compartments: blood, tumor, normal tissue, kidney, and liver. This model is characterized by a system of five ordinary differential equations using mass action kinetics, which describe uptake, intercompartmental transitions, and clearance rates. The ordinary differential equations were simultaneously solved and fit to experimental data using a genetic algorithm for optimization. The BD data show that both compounds have minimal distribution to organs at risk other than the kidney and liver. The PK parameter estimates had less than 5% error. From these data, ²²⁵Ac-Ahx showed larger and faster uptake in the liver. Both compounds had comparable uptake and clearance rates for other compartments. The BD and PK behavior for two targeted radiopharmaceuticals were investigated. The PK model fit the experimental data and provided insight into the kinetics of the compounds systematically.

Radioimmunotherapy for Prostate Cancer--Current Status and Future Possibilities

Prostate cancer (PCa) is one of the most common cancers in men and is the second leading cause of cancer-related deaths in the USA. In the United States, it is the second most frequently diagnosed cancer after skin cancer, and in Europe it is number one. According to the American Cancer Society, approximately 221,000 men in the United States would be diagnosed with PCa during 2015, and approximately 28,000 would die of the disease. According to the International Agency for Research on Cancer, approximately 345,000 men were diagnosed with PCa in Europe during 2012, and despite more emphasis placed on early detection through routine screening, 72,000 men died of the disease. Hence, the need for improved therapy modalities is of utmost importance. And targeted therapies based on radiolabeled specific antibodies or peptides are a very interesting and promising alternative to increase the therapeutic efficacy and overall chance of survival of these patients. There are currently several preclinical and some clinical studies that have been conducted, or are ongoing, to investigate the therapeutic efficacy and toxicity of radioimmunotherapy (RIT) against PCa. One thing that is lacking in a lot of these published studies is the dosimetry data, which are needed to compare results between the studies and the study locations. Given the complicated tumor microenvironment and overall complexity of RIT to PCa, old and new targets and targeting strategies like combination RIT and pretargeting RIT are being improved and assessed along with various therapeutic radionuclides candidates. Given alone or in combination with other therapies, these new and improved strategies and RIT tools further enhance the clinical response to RIT drugs in PCa, making RIT for PCa an increasingly practical clinical tool.

New Approaches for Modeling Radiopharmaceutical Pharmacokinetics Using Continuous Distributions of Rates

Radiopharmaceutical pharmacokinetics are usually approximated by sums of discrete first-order rates, using 3 or more parameters. We hypothesized that pharmacokinetic processes can be modeled even better by continuous probability distributions (CPD) of rates, using only 1-2 parameters.

METHODS

To test this hypothesis, we used biodistribution data for 188Re-labeled melanin-specific antibody in blood, kidneys, liver, bone marrow, and lungs of melanoma xenograft-bearing mice. We used 3 discrete-rate models (monoexponential, monoexponential with constant, and biexponential) and 4 CPD models (stretched-exponential, modified stretched-exponential, simplified versions of stretched-exponential, and modified stretched-exponential). They were compared by sample-size-corrected Akaike information criterion. Total time integrals of radioactivity were computed for each model and averaged across all models.

RESULTS

The ratio of weights of evidence for CPD versus discrete-rate models was high for blood (12.2) and lungs (2.7), almost unity (0.99) for bone marrow, and slightly lower for kidneys (0.81) and liver (0.73). In all organs or tissues except lungs, model-averaged time integrals were 12.7%-54.0% higher than biexponential model estimates.

CONCLUSION

Simple CPD models often outperform more complex discrete-rate models on pharmacokinetic data. Radioactivity time integrals are more robustly estimated by multimodel inference than using any single model.

Population-Based Modeling Improves Treatment Planning Before (90)Y-Labeled Anti-CD66 Antibody Radioimmunotherapy

For treatment planning in radioimmunotherapy (RIT), the accurate estimation of time-integrated activity coefficients (TIACs) is essential. To estimate the TIACs in RIT using (90)Y-labeled anti-CD66 antibodies, physiologically based pharmacokinetic (PBPK) models are advantageous. Further optimization in predicting therapeutic TIACs may be achieved by including population-specific parameters. Therefore, the aims of this work were (1) to estimate population parameters and (2) to show the effect of these parameters on prediction accuracy of therapeutic biodistributions. To estimate population values, a PBPK model was fitted to pretherapeutic (gamma camera and serum) and therapeutic (serum) measurements simultaneously using the standard two-stage (STS) and iterated two-stage (ITS) algorithms. Including the estimated population values as Bayesian information, the model parameters of each patient were fitted to pretherapeutic data only (simulating therapeutic TIACs). To validate the prediction accuracy of the therapeutic serum curve, the simulated and fitted TIACs were compared. Prediction accuracy expressed as relative deviation (RD) improved from RD=8%±16% to RD=0%±10% for STS and ITS, respectively. The authors demonstrated a method to estimate and apply population values for RIT using a PBPK model and population fitting. For (90)Y-labeled anti-CD66 antibodies, the prediction accuracy was substantially improved.

Treatment planning in molecular radiotherapy

In molecular radiotherapy a radionuclide or a radioactively labelled pharmaceutical is administered to the patient. Treatment planning therefore comprises the determination of activity to administer. This administered activity should maximize tumour cell sterilization while minimizing normal tissue damage. In this work we present different approaches that are frequently used for determining the suitable activity. These approaches may be cohort- based as in chemotherapy, or patient-specific using dosimetry based on individual biokinetics. The approaches are different with respect to the input complexity, the corresponding costs and - in consequence - the quality of the therapy. In addition, a general scheme for data collection and analysis is proposed. To develop an effective and safe treatment, elaborate data need to be obtained. The main challenges, however, are collecting these complex data and analyse them properly.

Dosimetric effectiveness of targeted radionuclide therapy based on a pharmacokinetic landscape

Assessment of targeted radionuclide therapy (TRT) agent effectiveness based on its pharmacokinetic (PK) properties could provide means to expedited agent development or its rejection. A broad PK model that predicts the relative effectiveness of TRT agents based on the relationship between their normal body (k(12), k(21)) and tumor (k(34), k(43)) PK parameters has been developed. A classic two-compartment open model decoupled from a tumor was used to represent the body. Analytically solved differential equations were used to develop a relationship that predicts TRT effectiveness. Various PK scenarios were created by pairing normal body PK parameters of 38 pharmaceuticals found in the literature with estimated tumor PK parameters. Each PK scenario resulted in a maximum permissible injected activity that limited the whole-body dose to 2 Gy and yielded a maximum delivered tumor dose. The model suggests that a k(34):k(43) ratio greater than 5 and a k(12):k(21) ratio less than 1 is effective at delivering doses that ensure sufficient solid tumor control. It was also shown that there is no direct relationship between tumor dose and acid dissociation constant (pK(a)), lipophilicity (log P), and fraction unbound (fu), which are important physicochemical properties. This study suggests that although effective TRT may be difficult to achieve for solid tumors, good TRT agents must have extremely desirable normal body PKs in conjunction with very high tumor retention. The developed PK TRT model could serve as a tool to compare the relative dosimetric effectiveness of existing TRT agents and novel TRT agents early in the developmental phase to potentially reject those that possess unfavorable PKs.

Pretargeted radioimmunotherapy for hematologic and other malignancies

Radioimmunotherapy (RIT) has emerged as one of the most promising treatment options, particularly for hematologic malignancies. However, this approach has generally been limited by a suboptimal therapeutic index (target-to-nontarget ratio) and an inability to deliver sufficient radiation doses to tumors selectively. Pretargeted RIT (PRIT) circumvents these limitations by separating the targeting vehicle from the subsequently administered therapeutic radioisotope, which binds to the tumor-localized antibody or is quickly excreted if unbound. A growing number of preclinical proof-of-principle studies demonstrate that PRIT is feasible and safe and provides improved directed radionuclide delivery to malignant cells compared with conventional RIT while sparing normal cells from nonspecific radiotoxicity. Early phase clinical studies corroborate these preclinical findings and suggest better efficacy and lesser toxicities in patients with hematologic and other malignancies. With continued research, PRIT-based treatment strategies promise to become cornerstones to improved outcomes for cancer patients despite their complexities.

An imaging-driven model for liposomal stability and circulation

Simultaneous labeling of the drug compartment and shell of delivery vehicles with optical and positron emission tomography (PET) probes is developed and employed to inform a hybrid physiologically based pharmacokinetic model. Based on time-dependent estimates of the concentration of these tracers within the blood pool, reticuloendothelial system (RES) and tumor interstitium, we compare the stability and circulation of long-circulating and temperature-sensitive liposomes. We find that rates of transport to the RES for long-circulating and temperature-sensitive particles are 0.046 and 0.19 h(-1), respectively. Without the application of exogenous heat, the rates of release from the long-circulating and temperature-sensitive particles circulating within the blood pool are 0.003 and 0.2 h(-1), respectively. Prolonged lifetime in circulation and slow drug release from liposomes result in a significantly greater drug area under the curve for the long-circulating particles. Future studies will couple these intrinsic parameters with exogenous heat-based release. Finally, we develop a transport constant for the transport of liposomes from the blood pool to the tumor interstitium, which is on the order of 0.01 h(-1) for the Met-1 tumor system.

Model selection for time-activity curves: the corrected Akaike information criterion and the F-test

Data analysis often requires a multi model approach, i.e. the best model or models are selected from a well chosen set of candidate models and subsequent parameter inference is conducted. The selection of the model or models which are best supported by the data can be accomplished using various criteria. The present work focuses on the comparison of two approaches namely the corrected Akaike information criterion (AICc) and the F-test for sparse data sets, which are common in medical research. The selection of the true model and the determination of relevant pharmacokinetic parameters as the clearance, the volume of distribution and the mean residence time are examined using Monte Carlo simulations with 10000 replications. The data (N = 10 per replication) are generated from a sum of two exponentials, which parameters were determined by fitting to time-concentration data of 111In labelled anti-CD66 antibody in blood serum. Four different normal distributed multiplicative statistical errors (0.05, 0.1, 0.15, 0.2) were examined. The set of candidate models consists of sums of up to 3 exponentials. Comparisons with two different model set sizes were conducted. All candidate models are fitted to the generated data and selected according to the AICc and the F-test. Both selection criteria perform well for our data. The selection frequency of functions of lower dimension increases proportionally to the statistical error for both criteria, while for higher errors, the AICc tends to choose a model of lower dimension more frequently than the F-test. In addition, the overfitted fraction decreases proportionally to the statistical error for both methods but selection frequency of function of higher dimension is larger using the F-test. The choice of the adequate model set is important for the positive effect of model averaging concerning the bias and the variability of the estimated parameters. It is in general assumed and has been confirmed in this study that parameter estimation using the AICc has clear advantages over the F-test.

Comparing time activity curves using the Akaike information criterion

The comparison of curves is a common task in many fields of science. Simply comparing the sums of squares or R(2) is not sufficient, and frequently used tests have many disadvantages. The basic idea of the presented method is turning the problem of comparing curves into a problem of model selection using the corrected Akaike Information Criterion. Here, this straightforward approach is applied for comparing curves using the example of (111)In- and (90)Y-labelled anti-CD66 antibody serum time activity data. As a result it is shown that for the investigated (111)In- and (90)Y-labelled anti-CD66 antibodies, the biokinetics between dosimetry and therapy are different with respect to the contribution of the second, longer half-life component. We advocate the use of the presented method rather than employing less advanced approaches for curve comparison.

BGRT: biologically guided radiation therapy-the future is fast approaching!

Rapid advances in functional and biological imaging, predictive assays, and our understanding of the molecular and cellular responses underpinning treatment outcomes herald the coming of the long-sought goal of implementing patient-specific biologically guided radiation therapy (BGRT) in the clinic. Biological imaging and predictive assays have the potential to provide patient-specific, three-dimensional information to characterize the radiation response characteristics of tumor and normal structures. Within the next decade, it will be possible to combine such information with advanced delivery technologies to design and deliver biologically conformed, individualized therapies in the clinic. The full implementation of BGRT in the clinic will require new technologies and additional research. However, even the partial implementation of BGRT treatment planning may have the potential to substantially impact clinical outcomes.

Biological optimization of heterogeneous dose distributions in systemic radiotherapy

The standard computational method developed for internal radiation dosimetry is the MIRD (medical internal radiation dose) formalism, based on the assumption that tumor control is given by uniform dose and activity distributions. In modern systemic radiotherapy, however, the need for full 3D dose calculations that take into account the heterogeneous distribution of activity in the patient is now understood. When information on nonuniform distribution of activity becomes available from functional imaging, a more patient specific 3D dosimetry can be performed. Application of radiobiological models can be useful to correlate the calculated heterogeneous dose distributions to the current knowledge on tumor control probability of a homogeneous dose distribution. Our contribution to this field is the introduction of a parameter, the F factor, already used by our group in studying external beam radiotherapy treatments. This parameter allows one to write a simplified expression for tumor control probability (TCP) based on the standard linear quadratic (LQ) model and Poisson statistics. The LQ model was extended to include different treatment regimes involving source decay, incorporating the repair "micro" of sublethal radiation damage, the relative biological effectiveness and the effective "waste" of dose delivered when repopulation occurs. The sensitivity of the F factor against radiobiological parameters (alpha, beta, micro) and the influence of the dose volume distribution was evaluated. Some test examples for 131I and 90Y labeled pharmaceuticals are described to further explain the properties of the F factor and its potential applications. To demonstrate dosimetric feasibility and advantages of the proposed F factor formalism in systemic radiotherapy, we have performed a retrospective planning study on selected patient case. F factor formalism helps to assess the total activity to be administered to the patient taking into account the heterogeneity in activity uptake and dose distribution, giving the same TCP of a homogeneous prescribed dose distribution. Animal studies and collection of standardized clinical data are needed to ascertain the effects of nonuniform dose distributions and to better assess the radiobiological input parameters of the model based on LQ model.

Blood Pharmacokinetics of Various Monoclonal Antibodies Labeled with a New Trifunctional Chelating Reagent for Simultaneous Conjugation with 1,4,7,10-Tetraazacyclododecane-N,N′,N″,N‴-Tetraacetic Acid and Biotin before Radiolabeling

PURPOSE

Knowledge of the blood pharmacokinetics of monoclonal antibodies is crucial in deciding the optimal time for starting the administration of a "clearing agent" or using a "clearing device." The primary purpose was to investigate whether the pharmacokinetics of various antibodies labeled with the same chelator and (111)In differed significantly after i.v. injection in immunocompetent rats. A new trifunctional chelator called "1033" containing a biotin and a radiometal chelation moiety is introduced, making it possible to use only one conjugation procedure for the antibody.

EXPERIMENTAL DESIGN

Sixty-five non-tumor-bearing rats were included and divided into four groups (I-IV). The blood pharmacokinetics was investigated for rituximab, BR96, and trastuzumab labeled with 1033 and (111)In (I-III). The whole-body activity and activity uptake in muscle, liver, and kidney, which might explain differences in the early pharmacokinetics in blood, were also measured. hMN14 labeled with another chelator [1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA)], but with the same radionuclide ((111)In-biotin-DOTA-hMN14), was studied (IV). The blood pharmacokinetics from another 15 tumor-bearing rats was compared with those of non-tumor-bearing rats (III) by injection of (111)In-1033-BR96.

RESULTS

No statistical difference was detected between the groups regarding the blood pharmacokinetics of rituximab, BR96, or trastuzumab. The pharmacokinetics and biodistribution of (111)In-biotin-DOTA-hMN14 exhibited a clear difference compared with others. There were no significant differences in the blood pharmacokinetics of (111)In-1033-BR96 between tumor-bearing rats and non-tumor-bearing rats.

CONCLUSIONS

Different antibodies labeled with the trifunctional chelator 1033 and (111)In did not exhibit different blood pharmacokinetics, which means that the pharmacokinetics could be predicted irrespective of the IgG1 antibody chosen. A small tumor burden did not change the pharmacokinetics of the radioimmunoconjugates.

Internal radionuclide therapy: The ULMDOS software for treatment planning

Before therapy with unsealed radionuclides, a dosimetry assessment must be performed for each patient. We present the interactive software tool ULMDOS, which facilitates dosimetric calculations, enhances traceability, and adequate documentation. ULMDOS is developed in IDL 6.1 (Interactive Data Language) under Windows XP/2000. First the patient data, the radiotracer data, and optionally urine and serum data are entered. After loading planar gamma camera images and drawing regions of interest, the residence times can be calculated using fits of the time activity data to exponential functions. Data can be saved in ASCII format for retrospective examination and further processing. ULMDOS allows one to process the dosimetric calculations within a standardized environment, spares the time-consuming transfer of data between different software tools, enables the documentation of ROI and raw data, and reduces intraindividual variability. ULMDOS satisfies the required conditions for traceability and documentation as a prerequisite to routine use in clinical settings.

The nonuniformity of antibody distribution in the kidney and its influence on dosimetry

The therapeutic efficacy of radiolabeled antibody fragments can be limited by nephrotoxicity, particularly when the kidney is the major route of extraction from the circulation. Conventional dose estimates in kidney assume uniform dose deposition, but we have shown increased antibody localization in the cortex after glomerular filtration. The purpose of this study was to measure the radioactivity in cortex relative to medulla for a range of antibodies and to assess the validity of the assumption of uniformity of dose deposition in the whole kidney and in the cortex for these antibodies with a range of radionuclides. Storage phosphor plate technology (radioluminography) was used to acquire images of the distributions of a range of antibodies of various sizes, labeled with 125I, in kidney sections. This allowed the calculation of the antibody concentration in the cortex relative to the medulla. Beta-particle point dose kernels were then used to generate the dose-rate distributions from 14C, 131I, 186Re, 32P and 90Y. The correlation between the actual dose-rate distribution and the corresponding distribution calculated assuming uniform antibody distribution throughout the kidney was used to test the validity of estimating dose by assuming uniformity in the kidney and in the cortex. There was a strong inverse relationship between the ratio of the radioactivity in the cortex relative to that in the medulla and the antibody size. The nonuniformity of dose deposition was greatest with the smallest antibody fragments but became more uniform as the range of the emissions from the radionuclide increased. Furthermore, there was a strong correlation between the actual dose-rate distribution and the distribution when assuming a uniform source in the kidney for intact antibodies along with medium- to long-range radionuclides, but there was no correlation for small antibody fragments with any radioisotope or for short-range radionuclides with any antibody. However, when the cortex was separated from the whole kidney, the correlation between the actual dose-rate distribution and the assumed dose-rate distribution, if the source was uniform, increased significantly. During radioimmunotherapy, the extent of nonuniformity of dose deposition in the kidney depends on the properties of the antibody and radionuclide. For dosimetry estimates, the cortex should be taken as a separate source region when the radiopharmaceutical is small enough to be filtered by the glomerulus.

Isothiocyanate-trigalactose: application for antibody-targeted delivery of diagnostic and therapeutic agents

Radiolabeled monoclonal antibodies (MAb) and MAb-streptavidin conjugates exhibit slow blood clearance which impedes radioimmunoimaging and radioimmunotherapy. To control blood clearance and lower background levels, lesion-specific targeting proteins can be modified with galactose derivatives for liver uptake via the hepatocyte galactose receptor. In this study, an isothiocyanate-trigalactose derivative (ITC-Tgal) designed for direct coupling to protein amino groups, was synthesized and characterized. In vitro experimentation demonstrated efficient conjugation of ITC-Tgal to streptavidin (SA) and MAb Fab fragment with a corresponding decrease in protein net charge. In vivo studies were conducted with radiolabeled ITC-Tgal modified and native SA and MAb Fab fragment. ITC-Tgal modified SA and Fab fragment exhibited increased blood clearance with the liver uptake and the rate of blood clearance controlled by the extent of ITC-Tgal modification.

Monoclonal antibody therapy for B cell non-Hodgkin's lymphomas: emerging concepts of a tumour-targeted strategy

Although much progress has been made in the understanding of the pathobiology of malignant lymphomas in recent years, progress in the treatment of patients with this diagnosis has been limited. Monoclonal antibody therapy is an innovative and promising concept in the treatment of malignant lymphoma, and the current status of this treatment is reviewed here. Phase I/II clinical trials have proven the high antilymphoma activity of antibody-based therapeutic strategies. Radioimmunoconjugates with myeloablative activity have induced response rates of between 80 and 100% in heavily pretreated patients. The chimeric monoclonal antibody IDEC-C2B8 has shown high antilymphoma activity in patients with relapsed follicular lymphoma with an overall response rate of up to 50%. The combination of the IDEC-C2B8 antibody with standard chemotherapy has shown encouraging results with no increase in toxicity compared with chemotherapy alone. The introduction of antibody therapy promises to open new perspectives in the treatment of patients with malignant lymphoma. Prospective randomised clinical trials will define the patient who will gain maximal benefit from antibody-based therapy.

Kinetics of 111In-labeled bleomycin in patients with brain tumors: compartmental vs. non-compartmental models

The kinetics of an indium-111 labeled bleomycin complex (111In-BLMC) after rapid intravenous injection in patients with brain tumors was quantified by using compartmental and non-compartmental models. The models were applied to data obtained from 10 glioma, one meningioma, and one adenocarcinoma brain metastasis patients. Blood and urine samples from all the patients and tumor samples from three patients were collected. The mean transit time of 111In-BLMC in the plasma pool was 14 +/- 7 min without and 1.8 +/- 0.6 h when accounting for recirculation, and 13 +/- 4 h in the total body pool. The mean plasma clearance of 111In-BLMC was 0.3 +/- 0.1 m/blood/min and the mean half-life in urine was 3.5 +/- 0.6 h. The mean transfer coefficients for the open three-compartmental model were: excretion from plasma = 0.02 +/- 0.01, from depot to plasma = (12 +/- 9)*10(-4), from plasma to depot = 0.01 +/- 0.01, from tumor to plasma = 0.39 +/- 0.19 and from plasma to tumor = 1.11 +/- 0.57, all in units minute-1. The mean turnover time from the tumor was 4.5 +/- 2.7 min and from the depot 20 +/- 8 h. It is concluded that both compartmental and non-compartmental models are sufficient to describe the kinetics of indium-111 labeled bleomycin complex. The non-compartmental model is more practical and to some extent more efficient in describing the in vivo behaviors of 111In-BLMC than the compartmental model. The compartmental model used provides estimates of both extraction and excretion from the plasma and tumor.

Pretargeting strategies for radio-immunoguided tumour localisation and therapy

The selective recognition of tumour cells by monoclonal antibodies, labelled with radioactive isotopes, for use in diagnosis and treatment, forms the basis of immunoscintigraphy, radio-immunoguided surgery and radio-immunotherapy. Research into the application of these systems has encountered multiple difficulties, most notably a low tumour to non-tumour ratio of radioactivity. The development of pretargeting systems, separating the individual steps of tumour cell targeting and the introduction of the radioactive label, have led to significant increments in tumour to non-tumour ratios and an improvement in diagnostic accuracy. Before pretargeting strategies are applied clinically, a thorough understanding of these systems is required and forms the backbone of this report. Clinical examples of early trials have already confirmed many of the theoretical advantages of pretargeting systems and new protocols are already being investigated.

Abdominal SPECT/MRI fusion applied to the study of splenic and hepatic uptake of radiolabeled thrombocytes and colloids

The importance of applying MRI (CT)/SPECT fusion in the abdominal and thoracic areas has been recognized in recent studies aiming at radionuclide therapy of cancer. According to our earlier results spleen and liver volume determination with different segmentation methods is inaccurate with SPECT alone. We therefore applied a SPECT/MRI registration procedure to the estimation of spleen and liver volumes and spleen/liver activity ratios in three male volunteers administered 111In-labeled thrombocytes and 99mTc-labeled colloids. The objectives of the study were to investigate if the uptake of thrombocytes in the spleen and liver can be measured more accurately when the anatomical borders of these organs are transferred from MRI to SPECT, and to test a SPECT/MRI registration method for improving three-dimensional dosimetry for radiotherapy treatment planning. A good correlation was found between spleen/liver activity ratios calculated from volumetric average activity per pixel values and from total volumetric counts derived from registered data but not from projection data. The average registration residual with this SPECT/MRI fusion method is approximately 1-2 cm in the abdominal area. Combining anatomical images with SPECT is therefore important for improving quantitative SPECT also in the abdomen.

Pharmacokinetic analysis of the microscopic distribution of enzyme-conjugated antibodies and prodrugs: comparison with experimental data

A mathematical model was developed to improve understanding of the biodistribution and microscopic profiles of drugs and prodrugs in a system using enzyme-conjugated antibodies as part of a two-step method for cancer treatment. The use of monoclonal antibodies alone may lead to heterogeneous uptake within the tumour tissue; the use of a second, low molecular weight agent may provide greater penetration into tumour tissue. This mathematical model was used to describe concentration profiles surrounding individual blood vessels within a tumour. From these profiles the area under the curve and specificity ratios were determined. By integrating these results spatially, average tissue concentrations were determined and compared with experimental results from three different systems in the literature; two using murine antibodies and one using humanised fusion proteins. The maximum enzyme conversion rate (Vmax) and the residual antibody concentration in the plasma and normal tissue were seen to be key determinants of drug concentration and drug-prodrug ratios in the tumour and other organs. Thus, longer time delays between the two injections, clearing the antibody from the blood stream and the use of 'weaker' enzymes (lower Vmax) will be important factors in improving this prodrug approach. Of these, the model found the effective clearance of the antibody outside of the tumour to be the most effective. The use of enzyme-conjugated antibodies may offer the following advantages over the bifunctional antibody-hapten system: (i) more uniform distribution of the active agent; (ii) higher concentrations possible for the active agent; and (iii) greater specificity (therapeutic index).

Radioimmunotherapy of solid cancers: A review

Depending on radionuclide characteristics, radioimmunotherapy (RIT) relies on radioactivity to destroy cells distant from immunotargeted cells. Therefore, even heterogeneous tumors (for antigen recognition) can be treated, because not all cells have to be targeted. Substantial complete response rates have been reported in patients with non-Hodgkin's lymphoma. Much more modest results have been reported for patients with bulky solid tumors, e.g. adenocarcinomas. The radiation doses delivered by targeting antibodies are generally too low to achieve major therapeutic responses. Dose escalation is limited by myelotoxicity, and higher doses need to be delivered to neoplasms less radiosensitive than lymphomas. Various trials for both systemic and regional RIT have been reported on. Intraperitoneal administration has been applied for colorectal and ovarian carcinomas. Our own results indicate that, e.g., intraperitoneal pseudomyxoma can be treated with RIT. Myelotoxicity can be reduced by anti-antibody-enhancement, 2- and 3-step strategies, bispecific monoclonal antibodies (MAbs), and extracorporeal immunoadsorption. The radionuclide has to be selected properly for each purpose; it can be a beta-emitter, e.g. I-131, Y-90, Re-188, Re-186, Lu-177 or Sm-153, an alpha-emitter At-211 or Bi-212 or an Auger-emitter, e.g. I-125, I-123. One major problem with RIT, besides slow penetration rate into tumor tissue and low tumor-to-normal tissue ratio, is the HAMA response, which can be partly avoided by the use of humanized MAbs and immunosuppression. However, RIT will be, because of all the recent developments, an important form of cancer management.

Development and clinical application of immunoassays for European adder (Vipera berus berus) venom and antivenom

An ovine affinity purified Fab antivenom was used in a clinical trial in Sweden to treat European adder (Vipera berus berus) envenoming. Immunoassays were developed to measure V. b. berus venom and antivenom concentrations in clinical samples to help assess the efficacy of treatment. A radioimmunoassay (RIA) was developed, optimized and validated to measure plasma levels of V. b. berus venom and compared with a conventional ELISA. Both showed a similar variation of zero binding in biological samples and the results obtained correlated closely. However, the ELISA was quicker and more sensitive (0.8 compared with 2 micrograms/litre). Before administration of antivenom, V. b. berus venom concentrations in plasma ranged from 10 to 53 micrograms/litre; 12 hr after the Fab infusion, no patient had measurable levels. However, two patients had low venom levels 24 hr after treatment. ELISA and RIA were also developed, optimized and used to measure concentrations of free Fab in plasma. There was a biexponential fall of Fab concentration with a fast distribution phase (t 1/2 = 0.9 hr) and a slower elimination phase (t 1/2 = 18 hr). The amount of Fab excreted in urine was low.

Recent developments in the radioimmunotherapy of cancer

Recent trials using radiolabeled monoclonal antibodies, particularly for hematologic neoplasms, have demonstrated the ability of these agents to destroy tumor cells safely and specifically. Advances in molecular biology, immunology and radiochemistry will allow the continued development of strategies to overcome the remaining obstacles to effective therapy with radioimmunoconjugates.