Pharmacokinetics, microscale distribution, and dosimetry of alpha-emitter-labeled anti-PD-L1 antibodies in an immune competent transgenic breast cancer model

The Effects of Radiation Dose Heterogeneity on the Tumor Microenvironment and Anti-Tumor Immunity

Radiotherapy elicits dose- and lineage-dependent effects on immune cell survival, migration, activation, and proliferation in targeted tumor microenvironments. Radiation also stimulates phenotypic changes that modulate the immune susceptibility of tumor cells. This has raised interest in using radiotherapy to promote greater response to immunotherapies. To clarify the potential of such combinations, it is critical to understand how best to administer radiation therapy to achieve activation of desired immunologic mechanisms. In considering the multifaceted process of priming and propagating anti-tumor immune response, radiation dose heterogeneity emerges as a potential means for simultaneously engaging diverse dose-dependent effects in a single tumor environment. Recent work in spatially fractionated external beam radiation therapy demonstrates the expansive immune responses achievable when a range of high to low dose radiation is delivered in a tumor. Brachytherapy and radiopharmaceutical therapies deliver inherently heterogeneous distributions of radiation that may contribute to immunogenicity. This review evaluates the interplay of radiation dose and anti-tumor immune response and explores emerging methodological approaches for investigating the effects of heterogeneous dose distribution on immune responses.

ImmunoPET imaging of TIGIT in the glioma microenvironment

Glioblastoma (GBM) is the most common primary malignant brain tumor. Currently, there are few effective treatment options for GBM beyond surgery and chemo-radiation, and even with these interventions, median patient survival remains poor. While immune checkpoint inhibitors (ICIs) have demonstrated therapeutic efficacy against non-central nervous system cancers, ICI trials for GBM have typically had poor outcomes. TIGIT is an immune checkpoint receptor that is expressed on activated T-cells and has a role in the suppression of T-cell and Natural Killer (NK) cell function. As TIGIT expression is reported as both prognostic and a biomarker for anti-TIGIT therapy, we constructed a molecular imaging agent, [89Zr]Zr-DFO-anti-TIGIT (89Zr-αTIGIT), to visualize TIGIT in preclinical GBM by immunoPET imaging. PET imaging and biodistribution analysis of 89Zr-αTIGIT demonstrated uptake in the tumor microenvironment of GBM-bearing mice. Blocking antibody and irrelevant antibody tracer studies demonstrated specificity of 89Zr-αTIGIT with significance at a late time point post-tracer injection. However, the magnitude of 89Zr-αTIGIT uptake in tumor, relative to the IgG tracer was minimal. These findings highlight the features and limitations of using 89Zr-αTIGIT to visualize TIGIT in the GBM microenvironment.

Evaluation of targeting αVβ3 in breast cancers using RGD peptide-based agents

Patients with HER2-positive and triple negative breast cancer (TNBC) are associated with increased risk to develop metastatic disease including reoccurring disease that is resistant to standard and targeted therapies. The αVβ3 has been implicated in BC including metastatic disease. The aims of this study were to investigate the potential of αVβ3-targeted peptides to deliver radioactive payloads to BC tumors expressing αVβ3 on the tumor cells or limited to the tumors' neovascular. Additionally, we aimed to assess the pharmacokinetic profile of the targeted α-particle therapy (TAT) agent [225Ac]Ac-DOTA-cRGDfK dimer peptide and the in vivo generated decay daughters. The expression of αVβ3 in a HER2-positive and a TNBC cell line were evaluated using western blot analysis. The pharmacokinetics of [111In]In-DOTA-cRGDfK dimer, a surrogate for the TAT-agent, was evaluated in subcutaneous mouse tumor models. The pharmacokinetic of the TAT-agent [225Ac]Ac-DOTA-cRGDfK dimer and its decay daughters were evaluated in healthy mice. Selective uptake of [111In]In-DOTA-cRGDfK dimer was shown in subcutaneous tumor models using αVβ3-positive tumor cells as well as αVβ3-negative tumor cells where the expression is limited to the neovasculature. Pharmacokinetic studies demonstrated rapid accumulation in the tumors with clearance from non-target organs. Dosimetric analysis of [225Ac]Ac-DOTA-cRGDfK dimer showed the highest radiation absorbed dose to the kidneys, which included the contributions from the free in vivo generated decay daughters. This study shows the potential of delivering radioactive payloads to BC tumors that have αVβ3 expression on the tumor cells as well as limited expression to the neovascular of the tumor. Furthermore, this work determines the radiation absorbed doses to normal organs/tissues and identified key organs that act as suppliers and receivers of the actinium-225 free in vivo generated α-particle-emitting decay daughters.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

A review of recent advancements in Actinium-225 labeled compounds and biomolecules for therapeutic purposes

In nuclear medicine, cancers that cannot be cured or can only be treated partially by traditional techniques like surgery or chemotherapy are killed by ionizing radiation as a form of therapeutic treatment. Actinium-225 is an alpha-emitting radionuclide that is highly encouraging as a therapeutic approach and more promising for targeted alpha therapy (TAT). Actinium-225 is the best candidate for tumor cells treatment and has physical characteristics such as high (LET) linear energy transfer (150 keV per μm), half-life (t1/2  = 9.92d), and short ranges (400-100 μm) which prevent the damage of normal healthy tissues. The introduction of various new radiopharmaceuticals and radioisotopes has significantly assisted the advancement of nuclear medicine. Ac-225 radiopharmaceuticals continuously demonstrate their potential as targeted alpha therapeutics. 225 Ac-labeled radiopharmaceuticals have confirmed their importance in medical and clinical areas by introducing [225 Ac]Ac-PSMA-617, [225 Ac]Ac-DOTATOC, [225 Ac]Ac-DOTA-substance-P, reported significantly improved response in patients with prostate cancer, neuroendocrine, and glioma, respectively. The development of these radiopharmaceuticals required a suitable buffer, incubation time, optimal pH, and reaction temperature. There is a growing need to standardize quality control (QC) testing techniques such as radiochemical purity (RCP). This review aims to summarize the development of the Ac-225 labeled compounds and biomolecules. The current state of their reported resulting clinical applications is also summarized as well.

Evaluation of the pharmacokinetics, dosimetry, and therapeutic efficacy for the α-particle-emitting transarterial radioembolization (αTARE) agent [225Ac]Ac-DOTA-TDA-Lipiodol® against hepatic tumors

BACKGROUND

The liver is a common site for metastatic disease for a variety of cancers, including colorectal cancer. Both primary and secondary liver tumors are supplied through the hepatic artery while the healthy liver is supplied by the portal vein. Transarterial radioembolization (TARE) using yttrium-90 glass or resin microspheres have shown promising results with reduced side-effects but have similar survival benefits as chemoembolization in patients with hepatocellular carcinoma (HCC). This highlights the need for new novel agents against HCC. Targeted alpha therapy (TAT) is highly potent treatment due to the short range (sparing adjacent normal tissue), and densely ionizing track (high linear energy transfer) of the emitted α-particles. The incorporation of α-particle-emitting radioisotopes into treatment of HCC has been extremely limited, with our recent publication pioneering the field of α-particle-emitting TARE (αTARE). This study focuses on an in-depth evaluation of the αTARE-agent [225Ac]Ac-DOTA-TDA-Lipiodol® as an effective therapeutic agent against HCC regarding pharmacokinetics, dosimetry, stability, and therapeutic efficacy.

RESULTS

[225Ac]Ac-DOTA-TDA was shown to be a highly stable with bench-top stability at ≥ 95% radiochemical purity (RCP) over a 3-day period and serum stability was ≥ 90% RCP over 5-days. The pharmacokinetic data showed retention in the tumor of [225Ac]Ac-DOTA-TDA-Lipiodol® and clearance through the normal organs. In addition, the tumor and liver acted as suppliers of the free daughters, which accumulated in the kidneys supplied via the blood. The dose limiting organ was the liver, and the estimated maximum tolerable activity based on the rodents whole-body weight: 728-3641 Bq/g (male rat), 396-1982 Bq/g (male mouse), and 453-2263 Bq/g (female mouse), depending on an RBE-value (range 1-5). Furthermore, [225Ac]Ac-DOTA-TDA-Lipiodol® showed significant improvement in survival for both the male and female mice (median survival 47-days) compared with controls (26-days untreated, and 33-35-days Lipiodol® alone).

CONCLUSIONS

This study shows that [225Ac]Ac-DOTA-TDA-Lipiodol® is a stable compound allowing for centralized manufacturing and distribution world-wide. Furthermore, the result of this study support the continue development of evaluation of the αTARE-agent [225Ac]Ac-DOTA-TDA-Lipiodol® as a potential treatment option for treating hepatic tumors.

Theranostic Surrogacy of [123I]NaI for Differentiated Thyroid Cancer Radionuclide Therapy

Precise dosimetry has gained interest for interpreting the response assessments of novel therapeutic radiopharmaceuticals, as well as for improving conventional radiotherapies such as the "one dose fits all" approach. Although radioiodine as same-element isotope theranostic pairs has been used for differentiated thyroid cancer (DTC), there are insufficient studies on the determination of its dosing regimen for personalized medicine and on extrapolating strategies for companion diagnostic radiopharmaceuticals. In this study, DTC xenograft mouse models were generated after validating iodine uptakes via sodium iodine symporter proteins (NIS) through in vitro assays, and theranostic surrogacy of companion radiopharmaceuticals was investigated in terms of single photon emission computed tomography (SPECT) imaging and voxel-level dosimetry. Following a Monte Carlo simulation, the hypothetical energy deposition/dose distribution images were produced as [¹²³I]NaI SPECT scans with the use of ¹³¹I ion source simulation, and dose rate curves were used to estimate absorbed dose. For the tumor, a peak concentration of 96.49 ± 11.66% ID/g occurred 2.91 ± 0.42 h after [¹²³I]NaI injection, and absorbed dose for ¹³¹I therapy was estimated as 0.0344 ± 0.0088 Gy/MBq. The absorbed dose in target/off-target tissues was estimated by considering subject-specific heterogeneous tissue compositions and activity distributions. Furthermore, a novel approach was proposed for simplifying voxel-level dosimetry and suggested for determining the minimal/optimal scan time points of surrogates for pretherapeutic dosimetry. When two scan time points were set to T_max and 26 h and the group mean half-lives were applied to the dose rate curves, the most accurate absorbed dose estimates were determined [-22.96, 2.21%]. This study provided an experimental basis to evaluate dose distribution and is expected hopefully to improve the challenging dosimetry process for clinical use.

PET/CT in Patients with Breast Cancer Treated with Immunotherapy

Significant advances in breast cancer (BC) treatment have been made in the last decade, including the use of immunotherapy and, in particular, immune checkpoint inhibitors that have been shown to improve the survival of patients with triple negative BC. This narrative review summarizes the studies supporting the use of immunotherapy in BC. Furthermore, the usefulness of 2-deoxy-2-[¹⁸F]fluoro-D-glucose (2-[¹⁸F]FDG) positron emission/computerized tomography (PET/CT) to image the tumor heterogeneity and to assess treatment response is explored, including the different criteria to interpret 2-[¹⁸F]FDG PET/CT imaging. The concept of immuno-PET is also described, by explaining the advantages of mapping treatment targets with a non-invasive and whole-body tool. Several radiopharmaceuticals in the preclinical phase are referred too, and, considering their promising results, translation to human studies is needed to support their use in clinical practice. Overall, this is an evolving field in BC treatment, despite PET imaging developments, the future trends also include expanding immunotherapy to early-stage BC and using other biomarkers.

Gamma counting protocols for the accurate quantification of 225Ac and 213Bi without the need for a secular equilibrium between parent and gamma-emitting daughter

BACKGROUND

Quantification of actinium-225 through gamma counter measurements, when there is no secular equilibrium between actinium-225 and its gamma emitting daughters bismuth-213 and/or francium-221, can provide valuable information regarding the possible relocation of recoiled daughters such that related radiotoxicity effects can be evaluated. This study proposes a multiple time-point protocol using the bismuth-213 photopeak with measurements before secular equilibrium between actinium-225 and bismuth-213, and a single time-point protocol using both the francium-221 and bismuth-213 photopeak while assuming secular equilibrium between actinium-225 and francium-221 but not between bismuth-213 and actinium-225.

RESULTS

Good agreement (i.e. 3% accuracy) was obtained when relying on a multiple time-points measurement of bismuth-213 to quantify both actinium-225 and excess of bismuth-213. Following scatter correction, actinium-225 can be accurately quantified using the francium-221 in a single time-point measurement within 3% of accuracy. The analysis performed on the stability data of [225Ac]Ac-DEPA and [225Ac]Ac-DOTA complexes, before secular equilibrium between bismuth-213 and actinium-225 was formed, revealed considerable amounts of unbound bismuth-213 (i.e. more than 90%) after 24 h of the radiolabeling most likely due to the recoiled daughter effect.

CONCLUSION

Both protocols were able to accurately estimate 225Ac-activities provided the francium-221 energy window was corrected for the down scatter of the higher-energy gamma-emissions by bismuth-213. This could prove beneficial to study the recoiled daughter effect and redistribution of free bismuth-213 by monitoring the accumulation or clearance of bismuth-213 in different tissues during biodistribution studies or in patient samples during clinical studies. On the other hand, the single gamma counter measurement protocol, although required a 30 min waiting time, is more time and cost efficient and therefore more appropriate for standardized quality control procedures of 225Ac-labeled radiopharmaceuticals.

Preliminary evaluation of alpha-emitting radioembolization in animal models of hepatocellular carcinoma

Hepatocellular carcinoma is the most common primary liver cancer and the fifth most frequently diagnosed cancer worldwide. Most patients with advanced disease are offered non-surgical palliative treatment options. This work explores the first alpha-particle emitting radioembolization for the treatment and monitoring of hepatic tumors. Furthermore, this works demonstrates the first in vivo simultaneous multiple-radionuclide SPECT-images of the complex decay chain of an [²²⁵Ac]Ac-labeled agent using a clinical SPECT system to monitor the temporal distribution. A DOTA chelator was modified with a lipophilic moiety and radiolabeled with the α-particle emitter Actinium-225. The resulting agent, [²²⁵Ac]Ac-DOTA-TDA, was emulsified in ethiodized oil and evaluated in vivo in mouse model and the VX2 rabbit technical model of liver cancer. SPECT imaging was performed to monitor distribution of the TAT agent and the free daughters. The [²²⁵Ac]Ac-DOTA-TDA emulsion was shown to retain within the HEP2G tumors and VX2 tumor, with minimal uptake within normal tissue. In the mouse model, significant improvements in overall survival were observed. SPECT-imaging was able to distinguish between the Actinium-225 agent (Francium-221) and the loss of the longer lived daughter, Bismuth-213. An α-particle emitting TARE agent is capable of targeting liver tumors with minimal accumulation in normal tissue, providing a potential therapeutic agent for the treatment of hepatocellular carcinoma as well as a variety of hepatic tumors. In addition, SPECT-imaging presented here supports the further development of imaging methodology and protocols that can be incorporated into the clinic to monitor Actinium-225-labeled agents.

Implications of physics, chemistry and biology for dosimetry calculations using theranostic pairs

Theranostics is an emerging paradigm that combines imaging and therapy in order to personalize patient treatment. In nuclear medicine, this is achieved by using radiopharmaceuticals that target identical molecular targets for both imaging (using emitted gamma rays) and radiopharmaceutical therapy (using emitted beta, alpha or Auger-electron particles) for the treatment of various diseases, such as cancer. If the therapeutic radiopharmaceutical cannot be imaged quantitatively, a “theranostic pair” imaging surrogate can be used to predict the absorbed radiation doses from the therapeutic radiopharmaceutical. However, theranostic dosimetry assumes that the pharmacokinetics and biodistributions of both radiopharmaceuticals in the pair are identical or very similar, an assumption that still requires further validation for many theranostic pairs. In this review, we consider both same-element and different-element theranostic pairs and attempt to determine if factors exist which may cause inaccurate dose extrapolations in theranostic dosimetry, either intrinsic (e.g. chemical differences) or extrinsic (e.g. injecting different amounts of each radiopharmaceutical) to the radiopharmaceuticals. We discuss the basis behind theranostic dosimetry and present common theranostic pairs and their therapeutic applications in oncology. We investigate general factors that could create alterations in the behavior of the radiopharmaceuticals or the quantitative accuracy of imaging them. Finally, we attempt to determine if there is evidence showing some specific pairs as suitable for theranostic dosimetry. We show that there are a variety of intrinsic and extrinsic factors which can significantly alter the behavior among pairs of radiopharmaceuticals, even if they belong to the same chemical element. More research is needed to determine the impact of these factors on theranostic dosimetry estimates and on patient outcomes, and how to correctly account for them.

A Novel Small Cyclic Peptide-Based 68Ga-Radiotracer for Positron Emission Tomography Imaging of PD-L1 Expression in Tumors

In the tumor microenvironment, programmed death protein 1 and programmed death protein ligand 1 (PD-L1) signaling pathways help tumors escape the immune system. We designed a gallium-68 (⁶⁸Ga)-labeled small-molecule peptide-targeting PD-L1 and used positron emission tomography/computed tomography (PET/CT) to detect and dynamically monitor the expression level of PD-L1 in tumors. S-Cyclo(ETSK)-SF-NH₂ (SETSKSF) is a cyclic peptide inhibitor comprising seven amino-acid residues. We connected it with the chelating agent DOTA, labeled DOTA-SETSKSF, with the short half-life nuclide Ga-68, and measured the stability of ⁶⁸Ga-2,2',2″-(10-(2-((S)-1-((3S,6S,9S,18S)-18-((S)-1-((S)-1-amino-1-oxo-3-henylpropan-2-ylamino)-3-hydroxy-1-oxopropan-2-ylcarbamoyl)-6-((R)-1-hydroxyethyl)-3-(hydroxymethyl)-2,5,8,12-tetraoxo-1,4,7,13-tetraazacyclooctadecan-9-ylamino)-3-ydroxy-1-oxopropan-2-ylamino)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (⁶⁸Ga-DOTA-SETSKSF) in normal saline (NS), phosphate-buffered saline (PBS), and fetal bovine serum (FBS) in vitro. We conducted the ⁶⁸Ga-DOTA-SETSKSF affinity test, cell-specific uptake experiments, time-combined experiments, western blotting, and laser confocal experiments to confirm the expression and localization of PD-L1 at the cell level and determine the uptake. Biodistribution and imaging experiments were performed using the H1975, B16F10, and A549 tumor models. ⁶⁸Ga-DOTA-SETSKSF was successfully synthesized, and the radiochemical purity was >99% after purification. The in vitro stability of ⁶⁸Ga-DOTA-SETSKSF was >95% in NS, PBS, and FBS at 37 °C after 4 h of incubation. Cell-binding experiments confirmed that ⁶⁸Ga-DOTA-SETSKSF exhibited high uptake in H1975 tumors with high PD-L1 expression and low uptake in A549 tumors with low PD-L1 expression. The clear half-life (T_1/2) of ⁶⁸Ga-DOTA-SETSKSF from the blood was 14.48 ± 3.26 min. The percentages of the injected dose per gram of tissue (%ID/g) for H1975 and A549 tumors were 5.29 ± 0.21 and 0.89 ± 0.10 at 1 h after injection, respectively. The H1975 tumor-to-muscle and tumor-to-blood ratios were 41.79 ± 5.81 and 4.75 ± 0.19 at 4 h, respectively. Apart from the H1975 tumor, the kidney and the bladder showed high accumulation because ⁶⁸Ga-DOTA-SETSKSF was excreted through the urinary system. PET/CT images showed high accumulation of ⁶⁸Ga-DOTA-SETSKSF in H1975 tumors and low uptake in A549 tumors, which was consistent with the results of biodistribution experiments. ⁶⁸Ga-DOTA-SETSKSF is convenient to prepare, has high stability, can be used to monitor the expression of PD-L1, and has an extremely high clinical application value.

Preclinical Evaluation of Podoplanin-Targeted Alpha-Radioimmunotherapy with the Novel Antibody NZ-16 for Malignant Mesothelioma

The prognosis of advanced mesothelioma is poor. Podoplanin (PDPN) is highly expressed in most malignant mesothelioma. This study aimed to evaluate the potential alpha-radioimmunotherapy (RIT) with a newly developed anti-PDPN antibody, NZ-16, compared with a previous antibody, NZ-12.

METHODS

The in vitro properties of radiolabeled antibodies were evaluated by cell binding and competitive inhibition assays using PDPN-expressing H226 mesothelioma cells. The biodistribution of ¹¹¹In-labeled antibodies was studied in tumor-bearing mice. The absorbed doses were estimated based on biodistribution data. Tumor volumes and body weights of mice treated with ⁹⁰Y- and ²²⁵Ac-labeled NZ-16 were measured for 56 days. Histologic analysis was conducted.

RESULTS

The radiolabeled NZ-16 specifically bound to H226 cells with higher affinity than NZ-12. The biodistribution studies showed higher tumor uptake of radiolabeled NZ-16 compared with NZ-12, providing higher absorbed doses to tumors. RIT with ²²⁵Ac- and ⁹⁰Y-labeled NZ-16 had a significantly higher antitumor effect than RIT with ⁹⁰Y-labeled NZ-12. ²²⁵Ac-labeled NZ-16 induced a larger amount of necrotic change and showed a tendency to suppress tumor volumes and prolonged survival than ⁹⁰Y-labeled NZ-16. There is no obvious adverse effect.

CONCLUSIONS

Alpha-RIT with the newly developed NZ-16 is a promising therapeutic option for malignant mesothelioma.

Nuclear imaging-guided PD-L1 blockade therapy increases effectiveness of cancer immunotherapy

Objectives

Strategies to improve the responsiveness of programmed death-1 (PD-1)/programmed death ligand-1 (PD-L1) checkpoint blockade therapy remain an essential topic in cancer immunotherapy. In this study, we developed a new radiolabeled nanobody-based imaging probe ^99mTc-MY1523 targeting PD-L1 for the enhanced therapeutic efficacy of PD-L1 blockade immunotherapy by the guidance of ^99mTc-MY1523 SPECT/CT imaging.

Methods

The binding affinity and specificity of nanobody MY1523 were measured in vitro. MY1523 was radiolabeled with ^99mTc by a site-specific transpeptidation of Sortase-A, and the biodistribution and single photon emission CT (SPECT)/CT were performed in mice bearing different tumors. We used interferon-γ (IFN-γ) as an intervention means to establish animal models with different levels of PD-L1 expression, then investigated the ability of ^99mTc-MY1523 SPECT/CT for the in vivo non-invasive measurement of PD-L1 expression in tumors. Finally, the PD-L1 blockade immunotherapies guided by ^99mTc-MY1523 SPECT/CT were carried out in MC-38, A20, and 4T1 tumor-bearing mouse models, followed by the testing of tumor infiltration T cells.

Results

MY1523 exhibited a high binding affinity and specificity to PD-L1 and had no competitive binding with the therapeutic antibody. ^99mTc-MY1523 was prepared with high specific activity and radiochemical purity. It was found that tumor PD-L1 expression was dynamically upregulated by IFN-γ intervention in MC-38, A20, and 4T1 tumor-bearing mouse models, as indicated by ^99mTc-MY1523 SPECT/CT. The PD-L1 blockade therapy initiated during the therapeutic time window determined by ^99mTc-MY1523 SPECT/CT imaging significantly enhanced the therapeutic efficacy in all animal models, while the tumor growth was effectively suppressed, and the survival time of mice was evidently prolonged. A correlation between dynamically upregulated PD-L1 expression and improved PD-L1 blockade therapy effectiveness was revealed, and the markedly increased infiltration of effector T cells into tumors was verified after the imaging-guided therapy.

Conclusion

Our results demonstrated that ^99mTc-MY1523 SPECT/CT allowed a real-time, quantitative and dynamic mapping of PD-L1 expression in vivo, and the imaging-guided PD-L1 blockade immunotherapy significantly enhanced the therapeutic efficacy. This strategy merits translation into clinical practice for the better management of combination therapies with radiotherapy or chemotherapy.

Overview of the Most Promising Radionuclides for Targeted Alpha Therapy: The "Hopeful Eight"

Among all existing radionuclides, only a few are of interest for therapeutic applications and more specifically for targeted alpha therapy (TAT). From this selection, actinium-225, astatine-211, bismuth-212, bismuth-213, lead-212, radium-223, terbium-149 and thorium-227 are considered as the most suitable. Despite common general features, they all have their own physical characteristics that make them singular and so promising for TAT. These radionuclides were largely studied over the last two decades, leading to a better knowledge of their production process and chemical behavior, allowing for an increasing number of biological evaluations. The aim of this review is to summarize the main properties of these eight chosen radionuclides. An overview from their availability to the resulting clinical studies, by way of chemical design and preclinical studies is discussed.

Targeted Alpha Therapy: Progress in Radionuclide Production, Radiochemistry, and Applications

This review outlines the accomplishments and potential developments of targeted alpha (α) particle therapy (TAT). It discusses the therapeutic advantages of the short and highly ionizing path of α-particle emissions; the ability of TAT to complement and provide superior efficacy over existing forms of radiotherapy; the physical decay properties and radiochemistry of common α-emitters, including ²²⁵Ac, ²¹³Bi, ²²⁴Ra, ²¹²Pb, ²²⁷Th, ²²³Ra, ²¹¹At, and ¹⁴⁹Tb; the production techniques and proper handling of α-emitters in a radiopharmacy; recent preclinical developments; ongoing and completed clinical trials; and an outlook on the future of TAT.

Quantifying PD-L1 Expression to Monitor Immune Checkpoint Therapy: Opportunities and Challenges

Simple Summary

Malignant cells hijack the regulatory roles of immune checkpoint proteins for immune evasion and survival. Therapeutics blocking those proteins can restore the balance of the immune system and lead to durable responses in cancer patients. Although a subset of patients derive benefit, there are few non-invasive technologies to guide and monitor those therapies to improve success rates. This is a review of the advancements in non-invasive methods for quantification of immune checkpoint protein programmed death ligand 1 expression, a biomarker detected by immunohistochemistry and widely used for guiding immune checkpoint therapy.

Abstract

Therapeutics targeting programmed death ligand 1 (PD-L1) protein and its receptor PD-1 are now dominant players in restoring anti-tumor immune responses. PD-L1 detection by immunohistochemistry (IHC) is emerging as a reproducible biomarker for guiding patient stratification for those therapies in some cancers. However, PD-L1 expression in the tumor microenvironment is highly complex. It is upregulated by aberrant genetic alterations, and is highly regulated at the transcriptional, posttranscriptional, and protein levels. Thus, PD-L1 IHC is inadequate to fully understand the relevance of PD-L1 levels in the whole body and their dynamics to improve therapeutic outcomes. Imaging technologies could potentially assist in meeting that need. Early clinical investigations show promising results in quantifying PD-L1 expression in the whole body by positron emission tomography (PET). Within this context, this review summarizes advancements in regulation of PD-L1 expression and imaging agents, and in PD-L1 PET for drug development, and discusses opportunities and challenges presented by these innovations for guiding immune checkpoint therapy (ICT).

Evaluating 225Ac and 177Lu Radioimmunoconjugates against Antibody-Drug Conjugates for Small-Cell Lung Cancer

Interest in the use of ²²⁵Ac for targeted alpha therapies has increased dramatically over the past few years, resulting in a multitude of new isotope production and translational research efforts. However, ²²⁵Ac radioimmunoconjugate (RIC) research is still in its infancy, with most prior experience in hematologic malignancies and only one reported preclinical solid tumor study using ²²⁵Ac RICs. In an effort to compare ²²⁵Ac RICs to other current antibody conjugates, a variety of RICs are tested against intractable small-cell lung cancer (SCLC). We directly compare, in vitro and in vivo, two promising candidates of each α or β^- category, ²²⁵Ac and ¹⁷⁷Lu, versus pyrrolobenzodiazepine (PBD) nonradioactive benchmarks. The monoclonal antibody constructs are targeted to either delta like 3 protein (DLL3), a recently discovered SCLC target, or CD46 as a positive control. An immunocompromised maximum tolerated dose assay is performed on NOD SCID mice, along with tumor efficacy proof-of-concept studies in vivo. We overview the conjugation techniques required to create serum-stable RICs and characterize and compare in vitro cell killing with RICs conjugated to nonspecific antibodies (huIgG1) with either native or site-specific thiol loci against tumor antigen DLL3-expressing and nonexpressing cell lines. Using patient-derived xenografts of SCLC onto NOD SCID mice, solid tumor growth was controlled throughout 3 weeks before growth appeared, in comparison to PBD conjugate controls. NOD SCID mice showed lengthened survival using ²²⁵Ac compared to ¹⁷⁷Lu RICs, and PBD dimers showed full tumor suppression with nine out of ten mice. The exploration of RICs on a variety of antibody-antigen systems is necessary to direct efforts in cancer research toward promising candidates. However, the anti-DLL3-RIC system with ²²⁵Ac and ¹⁷⁷Lu appears to be not as effective as the anti-DLL3-PBD counterpart in SCLC therapy with matched antibodies and portrays the challenges in both SCLC therapy as well as the specialized utility of RICs in cancer treatment.

Alpha radioimmunotherapy using 225Ac-proteus-DOTA for solid tumors - safety at curative doses

This is the initial report of an α-based pre-targeted radioimmunotherapy (PRIT) using ²²⁵Ac and its theranostic pair, ¹¹¹In. We call our novel tumor-targeting DOTA-hapten PRIT system "proteus-DOTA" or "Pr." Herein we report the first results of radiochemistry development, radiopharmacology, and stoichiometry of tumor antigen binding, including the role of specific activity, anti-tumor efficacy, and normal tissue toxicity with the Pr-PRIT approach (as α-DOTA-PRIT). A series of α-DOTA-PRIT therapy studies were performed in three solid human cancer xenograft models of colorectal cancer (GPA33), breast cancer (HER2), and neuroblastoma (GD2), including evaluation of chronic toxicity at ~20 weeks of select survivors. Methods: Preliminary biodistribution experiments in SW1222 tumor-bearing mice revealed that ²²⁵Ac could not be efficiently pretargeted with current DOTA-Bn hapten utilized for ¹⁷⁷Lu or ⁹⁰Y, leading to poor tumor uptake in vivo. Therefore, we synthesized Pr consisting of an empty DOTA-chelate for ²²⁵Ac, tethered via a short polyethylene glycol linker to a lutetium-complexed DOTA for picomolar anti-DOTA chelate single-chain variable fragment (scFv) binding. Pr was radiolabeled with ²²⁵Ac and its imaging surrogate, ¹¹¹In. In vitro studies verified anti-DOTA scFv recognition of [²²⁵Ac]Pr, and in vivo biodistribution and clearance studies were performed to evaluate hapten suitability and in vivo targeting efficiency. Results: Intravenously (i.v.) administered ²²⁵Ac- or ¹¹¹In-radiolabeled Pr in mice showed rapid renal clearance and minimal normal tissue retention. In vivo pretargeting studies show high tumor accumulation of Pr (16.71 ± 5.11 %IA/g or 13.19 ± 3.88 %IA/g at 24 h p.i. for [²²⁵Ac]Pr and [¹¹¹In]Pr, respectively) and relatively low uptake in normal tissues (all average ≤ 1.4 %IA/g at 24 h p.i.). Maximum tolerated dose (MTD) was not reached for either [²²⁵Ac]Pr alone or pretargeted [²²⁵Ac]Pr at administered activities up to 296 kBq/mouse. Single-cycle treatment consisting of α-DOTA-PRIT with either huA33-C825 bispecific anti-tumor/anti-DOTA-hapten antibody (BsAb), anti-HER2-C825 BsAb, or hu3F8-C825 BsAb for targeting GPA33, HER2, or GD2, respectively, was highly effective. In the GPA33 model, no complete responses (CRs) were observed but prolonged overall survival of treated animals was 42 d for α-DOTA-PRIT vs. 25 d for [²²⁵Ac]Pr only (P < 0.0001); for GD2, CRs (7/7, 100%) and histologic cures (4/7, 57%); and for HER2, CRs (7/19, 37%) and histologic cures (10/19, 56%) with no acute or chronic toxicity. Conclusions: [²²⁵Ac]Pr and its imaging biomarker [¹¹¹In]Pr demonstrate optimal radiopharmacologic behavior for theranostic applications of α-DOTA-PRIT. For this initial evaluation of efficacy and toxicity, single-cycle treatment regimens were performed in all three systems. Histologic toxicity was not observed, so MTD was not observed. Prolonged overall survival, CRs, and histologic cures were observed in treated animals. In comparison to RIT with anti-tumor IgG antibodies, [²²⁵Ac]Pr has a much improved safety profile. Ultimately, these data will be used to guide clinical development of toxicity and efficacy studies of [²²⁵Ac]Pr, with the goal of delivering massive lethal doses of radiation to achieve a high probability of cure without toxicity.

Evaluation of [225Ac]Ac-DOTA-anti-VLA-4 for targeted alpha therapy of metastatic melanoma

Very late antigen 4 (VLA-4; also called integrin α4β1) is overexpressed in melanoma tumor cells with an active role in tumor growth, angiogenesis, and metastasis, making VLA-4 a potential target for targeted alpha therapy (TAT).

METHODS

An anti-VLA-4 antibody was conjugated to DOTA for [²²⁵Ac]Ac-labeling and DTPA for [¹¹¹In]In-labeling. The resulting agents, [²²⁵Ac]Ac- or [¹¹¹In]In-labeled anti-VLA-4 were evaluated in vitro, including binding affinity, internalization, and colony formation assays as well as in vivo biodistribution studies. In addition, the therapeutic efficacy of [²²⁵Ac]Ac-DOTA-anti-VLA-4 was evaluated in a disseminated disease mouse model of melanoma.

RESULTS

[¹¹¹In]In-DTPA-anti-VLA-4 demonstrated high affinity for VLA-4 (K_d = 5.2 ± 1.6 nM). [²²⁵Ac]Ac-DOTA-anti-VLA-4 was labeled with an apparent molar activity of 3.5 MBq/nmol and > 95% radiochemical purity. Colony formation assays demonstrated a decrease in the surviving fraction of B16F10 cells treated with [²²⁵Ac]Ac-DOTA-anti-VLA-4 compared to control. Biodistribution studies demonstrated accumulation in the VLA-4-positive tumor and VLA-4 rich organs. Therapeutic efficacy studies demonstrated a significant increase in survival in mice treated with [²²⁵Ac]Ac-DOTA-anti-VLA-4 as compared to controls.

CONCLUSION

The work presented here demonstrated that [²²⁵Ac]Ac-DOTA-anti-VLA-4 was effective as a treatment in mice with disseminated disease, but potentially has dose limiting hematopoietic toxicity. Preliminary studies presented here also supported the potential to overcome this limitation by exploring a pre-loading or blocking dose strategy, to optimize the targeting vector to help minimize the absorbed dose to VLA-4 rich organs while maximizing the dose delivered to VLA-4-positive melanoma tumor cells.

Targeted alpha therapy: a critical review of translational dosimetry research with emphasis on actinium-225

This review provides a general overview of the current achievements and challenges in translational dosimetry for targeted alpha therapy (TAT). The concept of targeted radionuclide therapy (TRNT) is described with an overview of its clinical applicability and the added value of TAT is discussed. For TAT, we focused on actinium-225 (225Ac) as an example for alpha particle emitting radionuclides and their features, such as limited range within tissue and high linear energy transfer, which make alpha particle emissions more effective in targeted killing of tumour cells compared to beta radiation. Starting with the state-of-the-art dosimetry for TRNT and TAT, we then describe the challenges that still need to be met in order to move to a personalized dosimetry approach for TAT. Specifically for 225Ac, we discuss the recoiled daughter effect which may provoke significant damage to healthy tissue or organs and should be considered. Next, a broad overview is given of the pre-clinical research on 225Ac-TAT with an extensive description of tools which are only available in a pre-clinical setting and their added value. In addition, we review the preclinical biodistribution and dosimetry studies that have been performed on TAT-agents and more specifically of 225Ac and its multiple progeny, and describe their potential role to better characterize the pharmacokinetic (PK) profile of TAT-agents and to optimize the use of theranostic approaches for dosimetry. Finally, we discuss the support pre-clinical studies may provide in understanding dose-effect relationships, linking radiation dose quantities to biological endpoints and even moving away from macro- to microdosimetry. As such, the translation of pre-clinical findings may provide valuable information and new approaches for improved clinical dosimetry, thus paving the way to personalized TAT.

B7-H3-targeted Radioimmunotherapy of Human Cancer

BACKGROUND

Targeted Radioimmunotherapy (RIT) is an attractive approach to selectively localize therapeutic radionuclides to malignant cells within primary and metastatic tumors while sparing normal tissues from the effects of radiation. Many human malignancies express B7-H3 on the tumor cell surface, while expression on the majority of normal tissues is limited, presenting B7-H3 as a candidate target for RIT. This review provides an overview of the general principles of targeted RIT and discusses publications that have used radiolabeled B7-H3-targeted antibodies for RIT of cancer in preclinical or clinical studies.

METHODS

Databases including PubMed, Scopus, and Google Scholar were searched for publications through June 2018 using a combination of terms including "B7-H3", "radioimmunotherapy", "targeted", "radiotherapy", and "cancer". After screening search results for relevancy, ten publications were included for discussion.

RESULTS

B7-H3-targeted RIT studies to date range from antibody development and assessment of novel Radioimmunoconjugates (RICs) in animal models of human cancer to phase II/III trials in humans. The majority of clinical studies have used B7-H3-targeted RICs for intra- compartment RIT of central nervous system malignancies. The results of these studies have indicated high tolerability and favorable efficacy outcomes, supporting further assessment of B7-H3-targeted RIT in larger trials. Preclinical B7-H3-targeted RIT studies have also shown encouraging therapeutic outcomes in a variety of solid malignancies.

CONCLUSION

B7-H3-targeted RIT studies over the last 15 years have demonstrated feasibility for clinical development and support future assessment in a broader array of human malignancies. Future directions worthy of exploration include strategies that combine B7-H3- targeted RIT with chemotherapy or immunotherapy.

Development of Targeted Alpha Particle Therapy for Solid Tumors

Targeted alpha-particle therapy (TAT) aims to selectively deliver radionuclides emitting α-particles (cytotoxic payload) to tumors by chelation to monoclonal antibodies, peptides or small molecules that recognize tumor-associated antigens or cell-surface receptors. Because of the high linear energy transfer (LET) and short range of alpha (α) particles in tissue, cancer cells can be significantly damaged while causing minimal toxicity to surrounding healthy cells. Recent clinical studies have demonstrated the remarkable efficacy of TAT in the treatment of metastatic, castration-resistant prostate cancer. In this comprehensive review, we discuss the current consensus regarding the properties of the α-particle-emitting radionuclides that are potentially relevant for use in the clinic; the TAT-mediated mechanisms responsible for cell death; the different classes of targeting moieties and radiometal chelators available for TAT development; current approaches to calculating radiation dosimetry for TATs; and lead optimization via medicinal chemistry to improve the TAT radiopharmaceutical properties. We have also summarized the use of TATs in pre-clinical and clinical studies to date.

Tracers for non-invasive radionuclide imaging of immune checkpoint expression in cancer

Abstract

Immunotherapy with checkpoint inhibitors demonstrates impressive improvements in the treatment of several types of cancer. Unfortunately, not all patients respond to therapy while severe immune-related adverse effects are prevalent. Currently, patient stratification is based on immunotherapy marker expression through immunohistochemical analysis on biopsied material. However, expression can be heterogeneous within and between tumor lesions, amplifying the sampling limitations of biopsies. Analysis of immunotherapy target expression by non-invasive quantitative molecular imaging with PET or SPECT may overcome this issue. In this review, an overview of tracers that have been developed for preclinical and clinical imaging of key immunotherapy targets, such as programmed cell death-1, programmed cell death ligand-1, IDO1 and cytotoxic T lymphocyte-associated antigen-4 is presented. We discuss important aspects to consider when developing such tracers and outline the future perspectives of molecular imaging of immunotherapy markers.

Graphical abstract

Current techniques in immune checkpoint imaging and its potential for future applications

Biodistribution of a Radiolabeled Antibody in Mice as an Approach to Evaluating Antibody Pharmacokinetics

(1) Background: Monoclonal antibodies are used in the treatment of multiple conditions including cancer, autoimmune disorders, and infectious diseases. One of the initial steps in the selection of an antibody candidate for further pre-clinical development is determining its pharmacokinetics in small animal models. The use of mass spectrometry and other techniques to determine the fate of these antibodies is laborious and expensive. Here we describe a straightforward and highly reproducible methodology for utilizing radiolabeled antibodies for pharmacokinetics studies. (2) Methods: Commercially available bifunctional linker CHXA” and ¹¹¹Indium radionuclide were used. A melanin-specific chimeric antibody A1 and an isotype matching irrelevant control A2 were conjugated with the CHXA”, and then radiolabeled with ¹¹¹In. The biodistribution was performed at 4 and 24 h time points in melanoma tumor-bearing and healthy C57BL/6 female mice. (3) The biodistribution of the melanin-binding antibody showed the significant uptake in the tumor, which increased with time, and very low uptake in healthy melanin-containing tissues such as the retina of the eye and melanized skin. This biodistribution pattern in healthy tissues was very close to that of the isotype matching control antibody. (4) Conclusions: The biodistribution experiment allows us to assess the pharmacokinetics of both antibodies side by side and to make a conclusion regarding the suitability of specific antibodies for further development.

Radioimmunoimaging and targeting treatment in an immunocompetent murine model of triple-negative breast cancer using radiolabeled anti-programmed death-ligand 1 monoclonal antibody

The overall aim of this study was to evaluate whether iodine-131 radiolabeled monoclonal antibody (mAb) targeting programmed death-ligand 1 (PD-L1) can be used for imaging of PD-L1 expression noninvasively in vivo and playing synergistic effect combined with immunotherapy. Anti-PD-L1 mAb was radiolabeled with iodine-131 (¹³¹ I-PD-L1 mAb) and was characterized in vitro. Biodistribution and imaging in vivo were performed periodically. Therapy study was conducted in triple-negative breast cancer-bearing BALB/c mice. As results, the labeling efficiencies of ¹³¹ I-PD-L1 mAb reached 80% ± 3%, with radiochemical purity of 97% ± 1%. ¹³¹ I-PD-L1 mAb preserved the capacity to bind living PD-L1-expressing cells specifically in vitro. Tumor radioactivity uptake of ¹³¹ I-PD-L1 mAb was significantly higher than that of control groups. The xenografts were clearly imaged from 48 to 72 hours noninvasively after injection of ¹³¹ I-PD-L1 mAb, while the xenografts were not imaged in control groups. Tumor growth was significantly inhibited, and median survival time was remarkably prolonged in combination therapy group compared with control groups. It was concluded that ¹³¹ I-PD-L1 mAb can be a potential theranostic candidate for visualizing of PD-L1 expression noninvasively and performing synergistic therapy in carcinomas.

Noninvasive imaging of the PD-1:PD-L1 immune checkpoint: Embracing nuclear medicine for the benefit of personalized immunotherapy

Molecular imaging of the immune checkpoint receptor PD-1 and its ligand PD-L1 is increasingly investigated as a strategy to guide and monitor PD-1:PD-L1-targeted immune checkpoint therapy. We provide an overview of the current state-of-the-art on PD-1- and PD-L1-specific imaging agents for quantitative, real-time assessment of PD-1:PD-L1 expression in the tumor environment and discuss their potential for clinical translation.

Targeted and Nontargeted α-Particle Therapies

α-Particle irradiation of cancerous tissue is increasingly recognized as a potent therapeutic option. We briefly review the physics, radiobiology, and dosimetry of α-particle emitters, as well as the distinguishing features that make them unique for radiopharmaceutical therapy. We also review the emerging clinical role of α-particle therapy in managing cancer and recent studies on in vitro and preclinical α-particle therapy delivered by antibodies, other small molecules, and nanometer-sized particles. In addition to their unique radiopharmaceutical characteristics, the increased availability and improved radiochemistry of α-particle radionuclides have contributed to the growing recent interest in α-particle radiotherapy. Targeted therapy strategies have presented novel possibilities for the use of α-particles in the treatment of cancer. Clinical experience has already demonstrated the safe and effective use of α-particle emitters as potent tumor-selective drugs for the treatment of leukemia and metastatic disease.

212Pb-Labeled Antibody 225.28 Targeted to Chondroitin Sulfate Proteoglycan 4 for Triple-Negative Breast Cancer Therapy in Mouse Models

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer with a poor prognosis. There is a clinical need for effective, targeted therapy strategies that destroy both differentiated TNBC cells and TNBC cancer initiating cells (CICs), as the latter are implicated in the metastasis and recurrence of TNBC. Chondroitin sulfate proteoglycan 4 (CSPG4) is overexpressed on differentiated tumor cells and CICs obtained from TNBC patient specimens, suggesting that CSPG4 may be a clinically relevant target for the imaging and therapy of TNBC. The purpose of this study was to determine whether α-particle radioimmunotherapy (RIT) targeting TNBC cells using the CSPG4-specific monoclonal antibody (mAb) 225.28 as a carrier was effective at eliminating TNBC tumors in preclinical models. To this end, mAb 225.28 labeled with ²¹²Pb (²¹²Pb-225.28) as a source of α-particles for RIT was used for in vitro Scatchard assays and clonogenic survival assays with human TNBC cells (SUM159 and 2LMP) grown as adherent cells or non-adherent CIC-enriched mammospheres. Immune-deficient mice bearing orthotopic SUM159 or 2LMP xenografts were injected i.v. with the targeted (225.28) or irrelevant isotype-matched control (F3-C25) mAbs, labeled with ^99mTc, ¹²⁵I, or ²¹²Pb for in vivo imaging, biodistribution, or tumor growth inhibition studies. ²¹²Pb-225.28 bound to adherent SUM159 and 2LMP cells and to CICs from SUM159 and 2LMP mammospheres with a mean affinity of 0.5 nM. Nearly ten times more binding sites per cell were present on SUM159 cells and CICs compared with 2LMP cells. ²¹²Pb-225.28 was six to seven times more effective than ²¹²Pb-F3-C25 at inhibiting SUM159 cell and CIC clonogenic survival (p < 0.05). Radiolabeled mAb 225.28 showed significantly higher uptake than radiolabeled mAb F3-C25 in SUM159 and 2LMP xenografts (p < 0.05), and the uptake of ²¹²Pb-225.28 in TNBC xenografts was correlated with target epitope expression. ²¹²Pb-225.28 caused dose-dependent growth inhibition of SUM159 xenografts; 0.30 MBq ²¹²Pb-225.28 was significantly more effective than 0.33 MBq ²¹²Pb-F3-C25 at inhibiting tumor growth (p < 0.01). These results suggest that CSPG4-specific ²¹²Pb-225.28 is a useful reagent for RIT of CSPG4-expressing tumors, including metastatic TNBC.

Progress in Targeted Alpha-Particle Therapy. What We Learned about Recoils Release from In Vivo Generators

This review summarizes recent progress and developments as well as the most important pitfalls in targeted alpha-particle therapy, covering single alpha-particle emitters as well as in vivo alpha-particle generators. It discusses the production of radionuclides like ²¹¹At, ²²³Ra, ²²⁵Ac/²¹³Bi, labelling and delivery employing various targeting vectors (small molecules, chelators for alpha-emitting nuclides and their biomolecular targets as well as nanocarriers), general radiopharmaceutical issues, preclinical studies, and clinical trials including the possibilities of therapy prognosis and follow-up imaging. Special attention is given to the nuclear recoil effect and its impacts on the possible use of alpha emitters for cancer treatment, proper dose estimation, and labelling chemistry. The most recent and important achievements in the development of alpha emitters carrying vectors for preclinical and clinical use are highlighted along with an outlook for future developments.

Targeting Strategies for the Combination Treatment of Cancer Using Drug Delivery Systems

Cancer cells have characteristics of acquired and intrinsic resistances to chemotherapy treatment-due to the hostile tumor microenvironment-that create a significant challenge for effective therapeutic regimens. Multidrug resistance, collateral toxicity to normal cells, and detrimental systemic side effects present significant obstacles, necessitating alternative and safer treatment strategies. Traditional administration of chemotherapeutics has demonstrated minimal success due to the non-specificity of action, uptake and rapid clearance by the immune system, and subsequent metabolic alteration and poor tumor penetration. Nanomedicine can provide a more effective approach to targeting cancer by focusing on the vascular, tissue, and cellular characteristics that are unique to solid tumors. Targeted methods of treatment using nanoparticles can decrease the likelihood of resistant clonal populations of cancerous cells. Dual encapsulation of chemotherapeutic drug allows simultaneous targeting of more than one characteristic of the tumor. Several first-generation, non-targeted nanomedicines have received clinical approval starting with Doxil® in 1995. However, more than two decades later, second-generation or targeted nanomedicines have yet to be approved for treatment despite promising results in pre-clinical studies. This review highlights recent studies using targeted nanoparticles for cancer treatment focusing on approaches that target either the tumor vasculature (referred to as 'vascular targeting'), the tumor microenvironment ('tissue targeting') or the individual cancer cells ('cellular targeting'). Recent studies combining these different targeting methods are also discussed in this review. Finally, this review summarizes some of the reasons for the lack of clinical success in the field of targeted nanomedicines.