Enhanced retention of the alpha-particle-emitting daughters of Actinium-225 by liposome carriers

An overview of targeted alpha therapy

The effectiveness of targeted α-therapy (TAT) can be explained by the properties of α-particles. Alpha particles are helium nuclei and are ~8,000 times larger than β(-)-particles (electrons). When emitted from radionuclides that decay via an α-decay pathway, they release enormous amounts of energy over a very short distance. Typically, the range of α-particles in tissue is 50-100 μm and they have high linear energy transfer (LET) with a mean energy deposition of 100 keV/μm, providing a more specific tumor cell killing ability without damage to the surrounding normal tissues than β(-)-emitters. Due to these properties, the majority of pre-clinical and clinical trials have demonstrated that α-emitters such as (225)Ac, (211)At, (212)Bi, (213)Bi, (212)Pb, (223)Ra, and (227)Th are ideal for the treatment of smaller tumor burdens, micrometastatic disease, and disseminated disease. Even though these α-emitters have favorable properties, the development of TAT has been limited by high costs, unresolved chemistry, and limited availability of the radionuclides. To overcome these limitations, more potent isotopes, additional sources, and more efficient isotope production methods should be addressed. Furthermore, better chelation and labeling methods with the improvements of isotope delivery, targeting vehicles, molecular targets, and identification of appropriate clinical applications are still required.

Actinium-225 in targeted alpha-particle therapeutic applications

Alpha particle-emitting isotopes are being investigated in radioimmunotherapeutic applications because of their unparalleled cytotoxicity when targeted to cancer and their relative lack of toxicity towards untargeted normal tissue. Actinium- 225 has been developed into potent targeting drug constructs and is in clinical use against acute myelogenous leukemia. The key properties of the alpha particles generated by 225Ac are the following: i) limited range in tissue of a few cell diameters; ii) high linear energy transfer leading to dense radiation damage along each alpha track; iii) a 10 day halflife; and iv) four net alpha particles emitted per decay. Targeting 225Ac-drug constructs have potential in the treatment of cancer.

Cancer radioimmunotherapy

Targeting of radionuclides with antibodies, or radioimmunotherapy, has been an active field of research spanning nearly 50 years, evolving with advancing technologies in molecular biology and chemistry, and with many important preclinical and clinical studies illustrating the benefits, but also the challenges, which all forms of targeted therapies face. There are currently two radiolabeled antibodies approved for the treatment of non-Hodgkin lymphoma, but radioimmunotherapy of solid tumors remains a challenge. Novel antibody constructs, focusing on treatment of localized and minimal disease, and pretargeting are all promising new approaches that are currently under investigation.

"Smart" liposomal nanocontainers in biology and medicine

The perspectives of using liposomes for delivery of drugs to desired parts of the human body have been intensively investigated for more than 30 years. During this time many inventions have been suggested and different kinds of liposomal devices developed, and a number of them have reached the stages of preclinical or clinical trials. The latest techniques can be used to develop biocompatible nano-sized liposomal containers having some abilities of artificial intellect, such as the presence of sensory and responsive units. However, only a few have been clinically approved. Further improvements in this area depend on our knowledge of the interactions of drugs with the lipid bilayer of liposomes. Further studies on liposomal transport through the human body, their targeting of cells requiring therapeutic treatment, and finally, the development of techniques for controlled drug delivery to desired acceptors on cell surfaces or in cytoplasm are still required.

Recently identified and potential targets for colon cancer treatment

The systemic therapy for colorectal cancer has advanced from essentially a single, partially effective agent, 5-fluorouracil, to a combination of cytotoxics and antibodies offering increased survival. In addition to damage of DNA through agents, such as oxaliplatin and irinotecan, and inhibition of DNA replication, a promising approach involves modifying the control of gene expression, including epigenetic control. Modulation of invasion and metastasis should become increasingly important. Inhibition of growth-factor signaling with small-molecule drugs and antibodies can be a part of this effort. Further progress in the control of gene expression in colon cancer may be achieved with miRNAs and RNA interference if technical problems can be overcome. A number of genetic changes in colorectal cancer progression have been identified and offer targets for future therapy.

Nanotargeted radionuclides for cancer nuclear imaging and internal radiotherapy

Current progress in nanomedicine has exploited the possibility of designing tumor-targeted nanocarriers being able to deliver radionuclide payloads in a site or molecular selective manner to improve the efficacy and safety of cancer imaging and therapy. Radionuclides of auger electron-, α-, β-, and γ-radiation emitters have been surface-bioconjugated or after-loaded in nanoparticles to improve the efficacy and reduce the toxicity of cancer imaging and therapy in preclinical and clinical studies. This article provides a brief overview of current status of applications, advantages, problems, up-to-date research and development, and future prospects of nanotargeted radionuclides in cancer nuclear imaging and radiotherapy. Passive and active nanotargeting delivery of radionuclides with illustrating examples for tumor imaging and therapy are reviewed and summarized. Research on combing different modes of selective delivery of radionuclides through nanocarriers targeted delivery for tumor imaging and therapy offers the new possibility of large increases in cancer diagnostic efficacy and therapeutic index. However, further efforts and challenges in preclinical and clinical efficacy and toxicity studies are required to translate those advanced technologies to the clinical applications for cancer patients.

Immunoliposomal delivery of 213Bi for alpha-emitter targeting of metastatic breast cancer

Current treatment for late-stage metastatic breast cancer is largely palliative. alpha-Particles are highly potent, short-range radiation emissions capable of sterilizing individual cells with one to three traversals of the cell nucleus. The alpha-emitter, (213)Bi (T(1/2) = 45.6 min), was conjugated to a 100-nm diameter liposomal-CHX-A''-DTPA construct, upon which the rat HER2/neu reactive antibody, 7.16.4, was grafted. A conjugation time of 10 minutes was achieved giving a specific activity corresponding to 0.1 (213)Bi atom per liposome; stability in vitro and in vivo was confirmed. Efficacy in a rat/neu transgenic mouse model of metastatic mammary carcinoma was investigated. Three days after left cardiac ventricular injection of 10(5) rat HER-2/neu-expressing syngeneic tumor cells, macrophage-depleted Neu-N mice were treated by i.v. injection with (a) 19.2 MBq (520 muCi) of liposome-CHX-A''-DTPA-(213)Bi, (b) 19.2 MBq of liposome-CHX-A''-DTPA-(213)Bi-7.16.4, (c) 4.44 MBq (120 muCi) of (213)Bi-7.16.4, and (d) cold (nonradioactive) liposome-CHX-A''-DTPA-7.16.4 as control. Treatment with (a) increased median survival time to 34 days compared with 29 days for the untreated controls (P = 0.013) and 27 days for treated cold controls. Treatment with the radiolabeled antibody-conjugated liposome (b) increased median survival time to 38 days (P = 0.0002 relative to untreated controls). The radiolabeled antibody-treated group (c) gave a median survival of 39 days, which was similar to that for the radiolabeled antibody-conjugated liposome-treated group (P = 0.5). We have shown that the (213)Bi radiolabeled immunoliposomes are effective in treating early-stage micrometastases, giving median survival times similar to those obtained with antibody-mediated delivery of (213)Bi in this animal model.

Techniques for loading technetium-99m and rhenium-186/188 radionuclides into pre-formed liposomes for diagnostic imaging and radionuclide therapy

Liposomes can serve as carriers of radionuclides for diagnostic imaging and therapeutic applications. Herein, procedures are outlined for radiolabeling liposomes with the gamma-emitting radionuclide, technetium-99m ((99m)Tc), for non-invasive detection of disease and for monitoring the pharmacokinetics and biodistribution of liposomal drugs, and/or with therapeutic beta-emitting radionuclides, rhenium-186/188 ((186/188)Re), for radionuclide therapy. These efficient and practical liposome radiolabeling methods use a post-labeling mechanism to load (99m)Tc or (186/188)Re into pre-formed liposomes prepared in advance of the labeling procedure. For all liposome radiolabeling methods described, a lipophilic chelator is used to transport (99m)Tc or (186/188)Re across the lipid bilayer of the pre-formed liposomes. Once within the liposome interior, the pre-encapsulated glutathione or ammonium sulfate (pH) gradient provides for stable entrapment of the (99m)Tc and (186/188)Re within the liposomes. In the first method, (99m)Tc is transported across the lipid bilayer by the lipophilic chelator, hexamethylpropyleneamine oxime (HMPAO) and (99m)Tc-HMPAO becomes trapped by interaction with the pre-encapsulated glutathione within the liposomes. In the second method, (99m)Tc or (186/188)Re is transported across the lipid bilayer by the lipophilic chelator, N,N-bis(2-mercaptoethyl)-N',N'-diethylethylenediamine (BMEDA), and (99m)Tc-BMEDA or (186/188)Re-BMEDA becomes trapped by interaction with pre-encapsulated glutathione within the liposomes. In the third method, an ammonium sulfate (pH) gradient loading technique is employed using liposomes with an extraliposomal pH of 7.4 and an interior pH of 5.1. BMEDA, which is lipophilic at pH 7.4, serves as a lipophilic chelator for (99m)Tc or (186/188)Re to transport the radionuclides across the lipid bilayer. Once within the more acidic liposome interior, (99m)Tc/(186/188)Re-BMEDA complex becomes protonated and more hydrophilic, which results in stable entrapment of the (99m)Tc/(186/188)Re-BMEDA complex within the liposomes. Since many commercially available liposomal drugs use an ammonium sulfate (pH) gradient for drug loading, these liposomal drugs can be directly radiolabeled with (99m)Tc-BMEDA for non-invasive monitoring of tissue distribution during treatment or with (186/188)Re-BMEDA for combination chemo-radionuclide therapy.

Noninternalizing monoclonal antibodies are suitable candidates for 125I radioimmunotherapy of small-volume peritoneal carcinomatosis

We have previously shown that, in vitro, monoclonal antibodies (mAbs) labeled with the Auger electron emitter (125)I are more cytotoxic if they remain at the cell surface and do not internalize in the cytoplasm. Here, we assessed the in vivo biologic efficiency of internalizing and noninternalizing (125)I-labeled mAbs for the treatment of small solid tumors.

METHODS

Swiss nude mice bearing intraperitoneal tumor cell xenografts were injected with 37 MBq (370 MBq/mg) of internalizing (anti-HER1) (125)I-m225 or noninternalizing (anti-CEA) (125)I-35A7 mAbs at days 4 and 7 after tumor cell grafting. Nonspecific toxicity was assessed using the irrelevant (125)I-PX mAb, and untreated controls were injected with NaCl. Tumor growth was followed by bioluminescence imaging. Mice were sacrificed when the bioluminescence signal reached 4.5 x 10(7) photons/s. Biodistribution analysis was performed to determine the activity contained in healthy organs and tumor nodules, and total cumulative decays were calculated. These values were used to calculate the irradiation dose by the MIRD formalism.

RESULTS

Median survival (MS) was 19 d in the NaCl-treated group. Similar values were obtained in mice treated with unlabeled PX (MS, 24 d) and 35A7 (MS, 24 d) or with (125)I-PX mAbs (MS, 17 d). Conversely, mice treated with unlabeled or labeled internalizing m225 mAb (MS, 76 and 77 d, respectively) and mice injected with (125)I-35A7 mAb (MS, 59 d) showed a significant increase in survival. Irradiation doses were comparable in all healthy organs, independently from the mAb used, whereas in tumors the irradiation dose was 7.4-fold higher with (125)I-labeled noninternalizing than with internalizing mAbs. This discrepancy might be due to iodotyrosine moiety release occurring during the catabolism of internalizing mAbs associated with high turnover rate.

CONCLUSION

This study indicates that (125)I-labeled noninternalizing mAbs could be suitable for radioimmunotherapy of small solid tumors and that the use of internalizing mAbs should not be considered as a requirement for the success of treatments with (125)I Auger electrons.

Realizing the potential of the Actinium-225 radionuclide generator in targeted alpha particle therapy applications

Alpha particle-emitting isotopes have been proposed as novel cytotoxic agents for augmenting targeted therapy. Properties of alpha particle radiation such as their limited range in tissue of a few cell diameters and their high linear energy transfer leading to dense radiation damage along each alpha track are promising in the treatment of cancer, especially when single cells or clusters of tumor cells are targeted. Actinium-225 (225 Ac) is an alpha particle-emitting radionuclide that generates 4 net alpha particle isotopes in a short decay chain to stable 209 Bi, and as such can be described as an alpha particle nanogenerator. This article reviews the literature pertaining to the research, development, and utilization of targeted 225 Ac to potently and specifically affect cancer.

Heterogeneous domains and membrane permeability in phosphatidylcholine-phosphatidic acid rigid vesicles as a function of pH and lipid chain mismatch

Heterogeneous lipid membranes tuned by pH were evaluated at 37 degrees C in the form of PEGylated vesicles composed of lipid pairs with dipalmitoyl ( n = 16) and distearoyl ( n = 18) chain lengths. One lipid type was chosen to have the titratable moiety phosphatidic acid on its headgroup, and the other lipid type was chosen to have a phosphatidylcholine headgroup. The effect of pH on the formation of lipid heterogeneities and on membrane permeability was studied on vesicles composed of lipid pairs with matching and nonmatching chain lengths. The formation of lipid heterogeneities increases with decreasing pH in membranes composed of lipid pairs with either matching or nonmatching chain lengths. Increased permeability with decreasing pH was exhibited only by membranes composed of lipid pairs with nonmatching chain lengths. Permeability rates correlate strongly with the predicted extent of interfacial boundaries of heterogeneities, suggesting defective packing among nonmatching acyl chains of lipids. In heterogeneous mixtures with one lipid type in the fluid state ( n = 12), the dependence of membrane permeability on pH is weaker. In the presence of serum proteins, PEGylated gel-phase vesicles containing lipid pairs with nonmatching chain lengths exhibit faster release rates with decreasing pH compared to measured release rates in phosphate buffer, suggesting a second mechanism of formation of separated phases. PEGylated vesicles composed of lipid pairs with nonmatching chain lengths labeled with internalizing anti-HER2/neu antibodies that target overexpressed antigens on the surface of SKOV3-NMP2 ovarian cancer cells exhibit specific cancer cell targeting, followed by extensive internalization (more than 84% of bound vesicles) and fast release of contents intracellularly. These PEGylated vesicles composed of rigid membranes for long blood circulation times that exhibit pH-dependent release of contents intracellularly could become potent drug delivery carriers for the targeted therapy of solid tumors.

Enhanced loading efficiency and retention of 225Ac in rigid liposomes for potential targeted therapy of micrometastases

Targeted alpha-particle emitters are promising therapeutics for micrometastatic disease. Actinium-225 has a 10-day half-life and generates a total of four alpha-particles per parent decay rendering (225)Ac an attractive candidate for alpha-therapy. For cancer cells with low surface expression levels of molecular targets, targeting strategies of (225)Ac using radiolabeled carriers of low specific radioactivities (such as antibodies) may not deliver enough alpha-particle emitters at the targeted cancer cells to result in killing. We previously proposed and showed using passive (225)Ac entrapment that liposomes can stably retain encapsulated (225)Ac for long time periods, and that antibody-conjugated liposomes (immunoliposomes) with encapsulated (225)Ac can specifically target and become internalized by cancer cells. However, to enable therapeutic use of (225)Ac-containing liposomes, high activities of (225)Ac need to be stably encapsulated into liposomes. In this study, various conditions for active loading of (225)Ac in preformed liposomes (ionophore-type, encapsulated buffer solution, and loading time) were evaluated, and liposomes with up to 73 +/- 9% of the initial activity of (225)Ac (0.2-200 microCi) were developed. Retention of radioactive contents by liposomes was evaluated at 37 degrees C in phosphate buffer and in serum-supplemented media. The main fraction of released (225)Ac from liposomes occurs within the first two hours of incubation. Beyond this two hour point, the encapsulated radioactivity is released from liposomes slowly with an approximate half-life of the order of several days. In some cases, after 30 days, (225)Ac retention as high as 81 +/- 7% of the initially encapsulated radioactivity was achieved. The (225)Ac loading protocol was also applied to immunoliposome loading without significant loss of targeting efficacy. Liposomes with surface-conjugated antibodies that are loaded with (225)Ac overcome the limitations of low specific activity for molecular carriers and are expected to be therapeutically useful against tumor cells having a low antigen density.

Radionuclide carriers for targeting of cancer

This review describes strategies for the delivery of therapeutic radionuclides to tumor sites. Therapeutic approaches are summarized in terms of tumor location in the body, and tumor morphology. These determine the radionuclides of choice for suggested targeting ligands, and the type of delivery carriers. This review is not exhaustive in examples of radionuclide carriers for targeted cancer therapy. Our purpose is two-fold: to give an integrated picture of the general strategies and molecular constructs currently explored for the delivery of therapeutic radionuclides, and to identify challenges that need to be addressed. Internal radiotherapies for targeting of cancer are at a very exciting and creative stage. It is expected that the current emphasis on multidisciplinary approaches for exploring such therapeutic directions should enable internal radiotherapy to reach its full potential.