Targeted killing of virally infected cells by radiolabeled antibodies to viral proteins

Current status of mannose receptor-targeted drug delivery for improved anti-HIV therapy

In the pursuit of achieving better therapeutic outcomes in the treatment of HIV, innovative drug delivery strategies have been extensively explored. Mannose receptors, which are primarily found on macrophages and dendritic cells, offer promising targets for drug delivery due to their involvement in HIV pathogenesis. This review article comprehensively evaluates recent drug delivery system advancements targeting the mannose receptor. We have systematically described recent developments in creating and utilizing drug delivery platforms, including nanoparticles, liposomes, micelles, noisomes, dendrimers, and other nanocarrier systems targeted at the mannose receptor. These strategies aim to enhance drug delivery specificity, bioavailability, and therapeutic efficacy while decreasing off-target effects and systemic toxicity. Furthermore, the article delves into how mannose receptors and HIV interact, highlighting the potential for exploiting this interaction to enhance drug delivery to infected cells. The review covers essential topics, such as the rational design of nanocarriers for mannose receptor recognition, the impact of physicochemical properties on drug delivery performance, and how targeted delivery affects the pharmacokinetics and pharmacodynamics of anti-HIV agents. The challenges of these novel strategies, including immunogenicity, stability, and scalability, and future research directions in this rapidly growing area are discussed. The knowledge synthesis presented in this review underscores the potential of mannose receptor-based targeted drug delivery as a promising avenue for advancing HIV treatment. By leveraging the unique properties of mannose receptors, researchers can design drug delivery systems that cater to individual needs, overcome existing limitations, and create more effective and patient-friendly treatments in the ongoing fight against HIV/AIDS.

Soluble CD4 and low molecular weight CD4-mimetic compounds sensitize cells to be killed by anti-HIV cytotoxic immunoconjugates

IMPORTANCE

HIV infection can be effectively treated to prevent the development of AIDS, but it cannot be cured. We have attached poisons to anti-HIV antibodies to kill the infected cells that persist even after years of effective antiviral therapy. Here we show that the killing of infected cells can be markedly enhanced by the addition of soluble forms of the HIV receptor CD4 or by mimics of CD4.

Development of 225Ac Production from Low Isotopic Dilution 229Th

The promise of 225Ac targeted alpha therapies has been on the horizon for the last two decades. TerraPower Isotopes are uniquely suited to produce clinically relevant quantities of 225Ac through the decay of 229Th. Herein, a rapid processing scheme to isolate radionuclidic and radioisotopically pure 225Ac in good yield (98%) produced from 229Th that contains significant quantities of 228Th activity is described. The characterization of each step of the process is presented along with the detailed characterization of the resulting 225Ac isotopic starting material that will support the cancer research and development efforts.

Radioimmunotherapy for the treatment of infectious diseases: a comprehensive update

INTRODUCTION

Corona Virus Disease of 2019 (COVID-19) pandemic has renewed interest in monoclonal antibodies for treating infectious diseases. During last two decades experimental data has been accumulated showing the potential of radioimmunotherapy (RIT) of infectious diseases. In addition, COVID-19 pandemic has created a novel landscape for opportunistic fungal infections in post-COVID-19 patients resulting from severe immune suppression.

AREAS COVERED

We analyze recent results on targeting "pan-antigens" shared by fungal pathogens in mouse models and in healthy dogs; on developing RIT of prosthetic joint infections (PJI); examine RIT as potential human immunodeficiency virus (HIV) cure strategy and analyze its mechanisms and safety. Literature review was performed using PubMed and Google Scholar and includes relevant articles from 2000 to 2022.

EXPERT OPINION

Some of the RIT of infection applications can, hopefully, be moved into the clinic earlier than others after preclinical development: (1) RIT of opportunistic fungal infections might contribute to saving lives as current antifungal drugs do not work in severely immunocompromised patients; (2) RIT of patients with PJI. Success of RIT in these patients will allow to expand the application of RIT to other similarly vulnerable patients' populations such as cancer patients with weakened immune system and organ transplant recipients.

Development of an actinium-225 radioimmunoconjugate for targeted alpha therapy against SARS-CoV-2

Cytotoxicity was induced in cells mimicking SARS-CoV-2 infection by a 225Ac-radiolabeled antibody that targets the RBD2 of the spike protein.

Emerging Preclinical and Clinical Applications of Theranostics for Nononcological Disorders

Studies in nuclear medicine have shed light on molecular imaging and therapeutic approaches for oncological and nononcological conditions. Using the same radiopharmaceuticals for diagnosis and therapeutics of malignancies, the theranostics approach, has improved clinical management of patients. Theranostic approaches for nononcological conditions are recognized as emerging topics of research. This review focuses on preclinical and clinical studies of nononcological disorders that include theranostic strategies. Theranostic approaches are demonstrated as possible in the clinical management of infections and inflammations. There is an emerging need for randomized trials to specify the factors affecting validity and efficacy of theranostic approaches in nononcological diseases.

Antibody Conjugates for Targeted Therapy Against HIV-1 as an Emerging Tool for HIV-1 Cure

Although advances in antiretroviral therapy (ART) have significantly improved the life expectancy of people living with HIV-1 (PLWH) by suppressing HIV-1 replication, a cure for HIV/AIDS remains elusive. Recent findings of the emergence of drug resistance against various ART have resulted in an increased number of treatment failures, thus the development of novel strategies for HIV-1 cure is of immediate need. Antibody-based therapy is a well-established tool in the treatment of various diseases and the engineering of new antibody derivatives is expanding the realms of its application. An antibody-based carrier of anti-HIV-1 molecules, or antibody conjugates (ACs), could address the limitations of current HIV-1 ART by decreasing possible off-target effects, reduce toxicity, increasing the therapeutic index, and lowering production costs. Broadly neutralizing antibodies (bNAbs) with exceptional breadth and potency against HIV-1 are currently being explored to prevent or treat HIV-1 infection in the clinic. Moreover, bNAbs can be engineered to deliver cytotoxic or immune regulating molecules as ACs, further increasing its therapeutic potential for HIV-1 cure. ACs are currently an important component of anticancer treatment with several FDA-approved constructs, however, to date, no ACs are approved to treat viral infections. This review aims to outline the development of AC for HIV-1 cure, examine the variety of carriers and payloads used, and discuss the potential of ACs in the current HIV-1 cure landscape.

Diagnostic, Prognostic, and Therapeutic Use of Radiopharmaceuticals in the Context of SARS-CoV-2

The coronavirus disease 2019 (COVID-19) outbreak has devastated the healthcare systems and economies of over 200 countries in just a few months. The etiological agent of COVID-19, SARS-CoV-2, is a highly contagious virus that can be transmitted by asymptomatic and symptomatic carriers alike. While in vitro testing techniques have allowed for population-wide screening, prognostic tools are required to assess the disease severity and therapeutic response, contributing to improve the patient clinical outcomes. Moreover, no specific antiviral against COVID-19 exists at the time of publication, severely limiting treatment against the infection. Hence, there is an urgent clinical need for innovative therapeutic strategies that may contribute to manage the COVID-19 outbreak and prevent future pandemics. Herein, we critically examine recent diagnostic, prognostic, and therapeutic advancements for COVID-19 in the field of radiopharmaceuticals. First, we summarize the gold standard techniques used to diagnose COVID-19, including in vitro assays and imaging techniques, and then discuss how radionuclide-based nuclear imaging provides complementary information for prognosis and treatment management of infected patients. Second, we introduce new emerging types of radiotherapies that employ radioimmunoconjugates, which have shown selective cytotoxic response in oncological studies, and critically analyze how these compounds could be used as therapeutic agents against SARS-CoV-2. Finally, this Perspective further discusses the emerging applications of radionuclides to study the behavior of pulmonary SARS-CoV-2 aerosol particles.

Nuclear Medicine in Times of COVID-19: How Radiopharmaceuticals Could Help to Fight the Current and Future Pandemics

The emergence and global spread of COVID-19, an infectious disease caused by the novel coronavirus SARS-CoV-2, has resulted in a continuing pandemic threat to global health. Nuclear medicine techniques can be used for functional imaging of (patho)physiological processes at the cellular or molecular level and for treatment approaches based on targeted delivery of therapeutic radionuclides. Ongoing development of radiolabeling methods has significantly improved the accessibility of radiopharmaceuticals for in vivo molecular imaging or targeted radionuclide therapy, but their use for biosafety threats such as SARS-CoV-2 is restricted by the contagious nature of these agents. Here, we highlight several potential uses of nuclear medicine in the context of SARS-CoV-2 and COVID-19, many of which could also be performed in laboratories without dedicated containment measures. In addition, we provide a broad overview of experimental or repurposed SARS-CoV-2-targeting drugs and describe how radiolabeled analogs of these compounds could facilitate antiviral drug development and translation to the clinic, reduce the incidence of late-stage failures and possibly provide the basis for radionuclide-based treatment strategies. Based on the continuing threat by emerging coronaviruses and other pathogens, it is anticipated that these applications of nuclear medicine will become a more important part of future antiviral drug development and treatment.

Comparison of various radioactive payloads for a human monoclonal antibody to glycoprotein 41 for elimination of HIV-infected cells

BACKGROUND

cART has significantly improved the life expectancy of people living with HIV (PLWH). However, it fails to eliminate the long-lived reservoir of latent HIV-infected cells. Radioimmunotherapy (RIT) relies on antigen-specific monoclonal antibodies (mAbs) for targeted delivery of lethal doses of ionizing radiation to cells. Previously, we have demonstrated that human mAb 2556 against HIV gp41 conjugated with ²¹³Bismuth radioisotope (t_1/2 = 46 min, alpha-emitter) selectively killed HIV-infected cells. ²²⁵Actinium (t_1/2 = 9.92 d, alpha-emitter) and ¹⁷⁷Lutetium (t_1/2 = 6.7 d, beta-emitter) are two long-lived clinically proven radioisotopes for cancer treatment which might be more effective in killing infected cells systemically and in CNS.

METHODS

In this study we have conjugated 2556 mAb with ²¹³Bi, ²²⁵Ac and ¹⁷⁷Lu, and compared their ability to kill HIV-infected human peripheral blood mononuclear cells (PBMCs) and monocytes. PBMCs and monocytes from healthy donors were infected with HIV_p49.5 and treated in vitro with increasing concentrations of ²¹³Bi (4-20 μCi)-, ²²⁵Ac (20-100 nCi)- and ¹⁷⁷Lu (4-50 μCi)-2556 mAb.

RESULTS

After three days post-treatment of infected PBMCs and monocytes, ²¹³Bi- and ¹⁷⁷Lu-conjugated 2556 mAb reduced virus production measured by p24 level in a dose-dependent manner, whereas, ²²⁵Ac-2556 showed minimal effect. However, seven days post-treatment all three radioisotopes showed significantly more pronounced reduction of virus replication as compared to control labeled mAb with ²²⁵Ac-2556 showing the least non-specific killing.

CONCLUSION

These results indicate that RIT holds promise as a novel treatment option for the eradication of HIV-infected cells that merits further study in combination with cART and reactivation drugs.

Radioimmunotherapy of Blastomycosis in a Mouse Model With a (1→3)-β-Glucans Targeting Antibody

Invasive fungal infections (IFI) cause devastating morbidity and mortality, with the number of IFIs more than tripling since 1979. Our laboratories were the first to demonstrate that radiolabeled microorganism-specific monoclonal antibodies are highly effective for treatment of experimental fungal, bacterial and viral infections. Later we proposed to utilize surface expressed pan-antigens shared by major IFI-causing pathogens such as beta-glucans as RIT targets. Here we evaluated in vivo RIT targeting beta-glucan in Blastomyces dermatitidis which causes serious infections in immunocompromised and immunocompetent individuals and in companion dogs. B. dermatitidis cells were treated with the 400-2 antibody to (1→3)-β-glucans radiolabeled with the beta-emitter 177Lutetium (¹⁷⁷Lu) and alpha-emitter 213Bismuth (²¹³Bi) and the efficacy of cell kill was determined by colony forming units (CFUs). To determine the antigen-specific localization of the 400-2 antibody in vivo, C57BL6 mice were infected intratracheally with 2 × 10⁵B. dermatitidis cells and given ¹¹¹In-400-2 antibody 24 h later. To evaluate the killing of B. dermatitidis cells with RIT, intratracheally infected mice were treated with 150 μCi ²¹³Bi-400-2 and their lungs analyzed for CFUs 96 h post-infection. ²¹³Bi-400-2 proved to be more effective in killing B. dermatitidis cells in vitro than ¹⁷⁷Lu-400-2. Three times more ¹¹¹In-400-2 accumulated in the lungs of infected mice, than in the non-infected ones. ²¹³Bi-400-2 lowered the fungal burden in the lungs of infected mice more than 2 logs in comparison with non-treated infected controls. In conclusion, our results demonstrate the ability of an anti-(1-3)-beta-D-glucan antibody armed with an alpha-emitter ²¹³Bi to selectively kill B. dermatitidis cells in vitro and in vivo. These first in vivo results of the effectiveness of RIT targeting pan-antigens on fungal pathogens warrant further investigation.

Rhenium-188 Labeled Radiopharmaceuticals: Current Clinical Applications in Oncology and Promising Perspectives

Rhenium-188 (¹⁸⁸Re) is a high energy beta-emitting radioisotope with a short 16.9 h physical half-life, which has been shown to be a very attractive candidate for use in therapeutic nuclear medicine. The high beta emission has an average energy of 784 keV and a maximum energy of 2.12 MeV, sufficient to penetrate and destroy targeted abnormal tissues. In addition, the low-abundant gamma emission of 155 keV (15%) is efficient for imaging and for dosimetric calculations. These key characteristics identify ¹⁸⁸Re as an important therapeutic radioisotope for routine clinical use. Moreover, the highly reproducible on-demand availability of ¹⁸⁸Re from the ¹⁸⁸W/¹⁸⁸Re generator system is an important feature and permits installation in hospital-based or central radiopharmacies for cost-effective availability of no-carrier-added (NCA) ¹⁸⁸Re. Rhenium-188 and technetium-99 m exhibit similar chemical properties and represent a “theranostic pair.” Thus, preparation and targeting of ¹⁸⁸Re agents for therapy is similar to imaging agents prepared with ^99mTc, the most commonly used diagnostic radionuclide. Over the last three decades, radiopharmaceuticals based on ¹⁸⁸Re-labeled small molecules, including peptides, antibodies, Lipiodol and particulates have been reported. The successful application of these ¹⁸⁸Re-labeled therapeutic radiopharmaceuticals has been reported in multiple early phase clinical trials for the management of various primary tumors, bone metastasis, rheumatoid arthritis, and endocoronary interventions. This article reviews the use of ¹⁸⁸Re-radiopharmaceuticals which have been investigated in patients for cancer treatment, demonstrating that ¹⁸⁸Re represents a cost effective alternative for routine clinical use in comparison to more expensive and/or less readily available therapeutic radioisotopes.

Recent Research Trends on Bismuth Compounds in Cancer Chemoand Radiotherapy

Background:

Application of coordination chemistry in nanotechnology is a rapidly developing research field in medicine. Bismuth complexes have been widely used in biomedicine with satisfactory therapeutic effects, mostly in Helicobacter pylori eradication, but also as potential antimicrobial and anti-leishmanial agents. Additionally, in recent years, application of bismuth-based compounds as potent anticancer drugs has been studied extensively.

Methods:

Search for data connected with recent trends on bismuth compounds in cancer chemo- and radiotherapy was carried out using web-based literature searching tools such as ScienceDirect, Springer, Royal Society of Chemistry, American Chemical Society and Wiley. Pertinent literature is covered up to 2016.

Results:

In this review, based on 213 papers, we highlighted a number of current problems connected with: (i) characterization of bismuth complexes with selected thiosemicarbazone, hydrazone, and dithiocarbamate classes of ligands as potential chemotherapeutics. Literature results derived from 50 papers show that almost all bismuth compounds inhibit growth and proliferation of breast, colon, ovarian, lung, and other tumours; (ii) pioneering research on application of bismuth-based nanoparticles and nanodots for radiosensitization. Results show great promise for improvement in therapeutic efficacy of ionizing radiation in advanced radiotherapy (described in 36 papers); and (iii) research challenges in using bismuth radionuclides in targeted radioimmunotherapy, connected with choice of adequate radionuclide, targeting vector, proper bifunctional ligand and problems with 213Bi recoil daughters toxicity (derived from 92 papers).

Conclusion:

This review presents recent research trends on bismuth compounds in cancer chemo- and radiotherapy, suggesting directions for future research.

Principles of Immunotherapy: Implications for Treatment Strategies in Cancer and Infectious Diseases

The advances in cancer biology and pathogenesis during the past two decades, have resulted in immunotherapeutic strategies that have revolutionized the treatment of malignancies, from relatively non-selective toxic agents to specific, mechanism-based therapies. Despite extensive global efforts, infectious diseases remain a leading cause of morbidity and mortality worldwide, necessitating novel, innovative therapeutics that address the current challenges of increasing antimicrobial resistance. Similar to cancer pathogenesis, infectious pathogens successfully fashion a hospitable environment within the host and modulate host metabolic functions to support their nutritional requirements, while suppressing host defenses by altering regulatory mechanisms. These parallels, and the advances made in targeted therapy in cancer, may inform the rational development of therapeutic interventions for infectious diseases. Although "immunotherapy" is habitually associated with the treatment of cancer, this review accentuates the evolving role of key targeted immune interventions that are approved, as well as those in development, for various cancers and infectious diseases. The general features of adoptive therapies, those that enhance T cell effector function, and ligand-based therapies, that neutralize or eliminate diseased cells, are discussed in the context of specific diseases that, to date, lack appropriate remedial treatment; cancer, HIV, TB, and drug-resistant bacterial and fungal infections. The remarkable diversity and versatility that distinguishes immunotherapy is emphasized, consequently establishing this approach within the armory of curative therapeutics, applicable across the disease spectrum.

Pharmaceutical Approaches to HIV Treatment and Prevention

Human immunodeficiency virus (HIV) infection continues to pose a major infectious disease threat worldwide. It is characterized by the depletion of CD4+ T cells, persistent immune activation, and increased susceptibility to secondary infections. Advances in the development of antiretroviral drugs and combination antiretroviral therapy have resulted in a remarkable reduction in HIV-associated morbidity and mortality. Antiretroviral therapy (ART) leads to effective suppression of HIV replication with partial recovery of host immune system and has successfully transformed HIV infection from a fatal disease to a chronic condition. Additionally, antiretroviral drugs have shown promise for prevention in HIV pre-exposure prophylaxis and treatment as prevention. However, ART is unable to cure HIV. Other limitations include drug-drug interactions, drug resistance, cytotoxic side effects, cost, and adherence. Alternative treatment options are being investigated to overcome these challenges including discovery of new molecules with increased anti-viral activity and development of easily administrable drug formulations. In light of the difficulties associated with current HIV treatment measures, and in the continuing absence of a cure, the prevention of new infections has also arisen as a prominent goal among efforts to curtail the worldwide HIV pandemic. In this review, the authors summarize currently available anti-HIV drugs and their combinations for treatment, new molecules under clinical development and prevention methods, and discuss drug delivery formats as well as associated challenges and alternative approaches for the future.

Radioimmunotherapy as a Novel Approach in HIV, Bacterial, and Fungal Infectious Diseases

In the past several decades, many antimicrobial agents have been used in treating different fungal, bacterial, and viral infections. However, these agents have faced challenges such as pronounced side-effect profiles and pathogen resistance. In addition, a cure for many chronic infections such as human immunodeficiency virus (HIV) has not been achieved, and the incidence of opportunistic infections in immunocompromised patients has increased significantly in the past decades. Therefore, an alternative strategy for combating these infections is needed. Radioimmunotherapy (RIT) has been proposed to be a valuable tool in the management of such infections. The side-effects associated with RIT are minimal as the targeted antigens are only expressed on microbial or infected cells. RIT demonstrated impressive potency in eradicating pathogens in animal models and patient samples. Cryptococcus neoformans, HIV, and Bacillus anthracis are few examples of infections for which RIT has been an effective treatment using radionuclides such as bismuth-213 (²¹³Bi) or rhenium-188 (¹⁸⁸Re).

Pharmacokinetic and Pharmacodynamic Considerations for the Use of Monoclonal Antibodies in the Treatment of Bacterial Infections

Antibiotic-resistant bacterial pathogens are increasingly implicated in hospital- and community-acquired infections. Recent advances in monoclonal antibody (mAb) production and engineering have led to renewed interest in the development of antibody-based therapies for treatment of drug-resistant bacterial infections. Currently, there are three antibacterial mAb products approved by the Food and Drug Administration (FDA) and at least nine mAbs are in clinical trials. Antibacterial mAbs are typically developed to kill bacteria or to attenuate bacterial pathological activity through neutralization of bacterial toxins and virulence factors. Antibodies exhibit distinct pharmacological mechanisms from traditional antimicrobials and, hence, cross-resistance between small molecule antimicrobials and antibacterial mAbs is unlikely. Additionally, the long biological half-lives typically found for mAbs may allow convenient dosing and vaccine-like prophylaxis from infection. However, the high affinity of mAbs and the involvement of the host immune system in their pharmacological actions may lead to complex and nonlinear pharmacokinetics and pharmacodynamics. In this review, we summarize the pharmacokinetics and pharmacodynamics of the FDA-approved antibacterial mAbs and those are currently in clinical trials. Challenges in the development of antibacterial mAbs are also discussed.

Transcriptomic Analysis Implicates the p53 Signaling Pathway in the Establishment of HIV-1 Latency in Central Memory CD4 T Cells in an In Vitro Model

The search for an HIV-1 cure has been greatly hindered by the presence of a viral reservoir that persists despite antiretroviral therapy (ART). Studies of HIV-1 latency in vivo are also complicated by the low proportion of latently infected cells in HIV-1 infected individuals. A number of models of HIV-1 latency have been developed to examine the signaling pathways and viral determinants of latency and reactivation. A primary cell model of HIV-1 latency, which incorporates the generation of primary central memory CD4 T cells (TCM), full-length virus infection (HIVNL4-3) and ART to suppress virus replication, was used to investigate the establishment of HIV latency using RNA-Seq. Initially, an investigation of host and viral gene expression in the resting and activated states of this model indicated that the resting condition was reflective of a latent state. Then, a comparison of the host transcriptome between the uninfected and latently infected conditions of this model identified 826 differentially expressed genes, many of which were related to p53 signaling. Inhibition of the transcriptional activity of p53 by pifithrin-α during HIV-1 infection reduced the ability of HIV-1 to be reactivated from its latent state by an unknown mechanism. In conclusion, this model may be used to screen latency reversing agents utilized in shock and kill approaches to cure HIV, to search for cellular markers of latency, and to understand the mechanisms by which HIV-1 establishes latency.

Combination of Antiretroviral Drugs and Radioimmunotherapy Specifically Kills Infected Cells from HIV-Infected Individuals

Eliminating virally infected cells is an essential component of any HIV eradication strategy. Radioimmunotherapy (RIT), a clinically established method for killing cells using radiolabeled antibodies, was recently applied to target HIV-1 gp41 antigen expressed on the surface of infected cells. Since gp41 expression by infected cells is likely downregulated in patients on antiretroviral therapy (ART), we evaluated the ability of RIT to kill ART-treated infected cells using both in vitro models and lymphocytes isolated from HIV-infected subjects. Human peripheral blood mononuclear cells (PBMCs) were infected with HIV and cultured in the presence of two clinically relevant ART combinations. Scatchard analysis of the 2556 human monoclonal antibody to HIV gp41 binding to the infected and ART-treated cells demonstrated sufficient residual expression of gp41 on the cell surface to warrant subsequent RIT. This is the first time the quantification of gp41 post-ART is being reported. Cells were then treated with Bismuth-213-labeled 2556 antibody. Cell survival was quantified by Trypan blue and residual viremia by p24 ELISA. Cell surface gp41 expression was assessed by Scatchard analysis. The experiments were repeated using PBMCs isolated from blood specimens obtained from 15 HIV-infected individuals: 10 on ART and 5 ART-naïve. We found that ²¹³Bi-2556 killed ART-treated infected PBMCs and reduced viral production to undetectable levels. ART and RIT co-treatment was more effective at reducing viral load in vitro than either therapy alone, indicating that gp41 expression under ART was sufficient to allow ²¹³Bi-2556 to deliver cytocidal doses of radiation to infected cells. This study provides proof of concept that ²¹³Bi-2556 may represent an innovative and effective targeting method for killing HIV-infected cells treated with ART and supports continued development of ²¹³Bi-2556 for co-administration with ART toward an HIV eradication strategy.

A fully human antibody to gp41 selectively eliminates HIV-infected cells that transmigrated across a model human blood brain barrier

OBJECTIVE

Many HIV patients on combined antiretroviral therapy exhibit HIV-associated neurocognitive disorders because the brain becomes a viral reservoir. There is a need for therapeutics that can enter the central nervous system (CNS) and eradicate the virus.

DESIGN

Radiolabeled human mAb 2556 to HIV gp41 selectively kills HIV-infected cells in vivo and in vitro. Here we tested the ability of 213Bi-2556 to cross a tissue culture model of the human blood brain barrier and kill HIV-infected peripheral blood mononuclear cells (PBMCs) and monocytes on the CNS side of the barrier.

METHODS

2556 mAb isoelectric point was determined with isoelectric focusing. The ability of radiolabeled 2556 to penetrate through the barrier was studied by adding it to the upper chamber of the barriers and its penetration into the CNS side was followed for 5 h. To assess the ability of Bi-2556 to kill the HIV-infected cells on the CNS side of barrier, the HIV-infected and uninfected PBMCs and monocytes were allowed to transmigrate across the barriers overnight followed by application of Bi-2556 or control mAb Bi-1418 to the top of the barrier. Killing of cells was measured by TUNEL and Trypan blue assays. The barriers were examined by confocal microscopy for overt damage.

RESULTS

The isoelectric point of Bi-2556 was 9.6 enabling its penetration through the barrier by transcytosis. Bi-2556 killed significantly more transmigrated HIV-infected cells in comparison to Bi-1418 and uninfected cells. No overt damage to barriers was observed.

CONCLUSION

We demonstrated that Bi-2556 mAb crossed an in-vitro human blood brain barrier and specifically killed transmigrated HIV-infected PBMCs and monocytes without overt damage to the barrier.

Radiolabeled Antibodies for Therapy of Infectious Diseases

Novel approaches to the treatment of infectious diseases are urgently needed. This need has resulted in renewing the interest in antibodies for therapy of infectious diseases. Radioimmunotherapy (RIT) is a cancer treatment modality that utilizes radiolabeled monoclonal antibodies. During the last decade we have translated RIT into the field of experimental fungal, bacterial, and HIV infections. In addition, successful proof of principle experiments with radiolabeled pan-antibodies that bind to antigens shared by major pathogenic fungi have been performed in vitro. The armamentarium of pan-antibodies would result in reducing our dependence on microorganism-specific antibodies and thus would speed up the development of RIT for infections. We believe that the time is ripe for deploying RIT in the clinic to combat infectious diseases.

Potential of Radiation-Induced Cellular Stress for Reactivation of Latent HIV-1 and Killing of Infected Cells

The use of highly active antiretroviral therapy against HIV-1 for last two decades has reduced mortality of patients through extension of nonsymptomatic phase of infection. However, HIV-1 can be preserved in long-lived resting CD4(+) T cells, which form a viral reservoir in infected individuals, and potentially in macrophages and astrocytes. Reactivation of viral replication is critical since the host immune response in combination with antiretroviral therapy may eradicate the virus (shock and kill strategy). In this opinion piece, we consider potential application of therapeutic doses of irradiation, the well-known and effective stress signal that induces DNA damage and activates cellular stress response, to resolve two problems: activate HIV-1 replication and virion production in persistent reservoirs under cART and deplete infected cells through selective cell killing using DNA damage responses.

Advancing the HIV cure agenda: the next 5 years

PURPOSE OF REVIEW

To explore how ethical considerations, improved diagnostics and data from clinical trials might see the lowering of some of the barriers blocking a cure for HIV infection over the next 5 years.

RECENT FINDINGS

Despite the recent well publicized but eventually disappointing case reports, there remains only one successful HIV cure, the 'Berlin patient'. We will review the data suggesting that more potent agents might achieve in-vivo viral activation and explore the tantalizing phenomenon of 'posttreatment control' following treatment in primary HIV infection. We will also explore how new assays and novel interventions might move the field forward.

SUMMARY

There is a need for new agents that can be safely tested to impact the viral reservoir, a more meaningful understanding of how to assay patient samples, and research into mechanisms behind how the reservoir is established and impacted by therapy. With HIV+ve individuals responding so well to antiretroviral therapy, new trials must be tested hand-in-hand with guidance from patient representatives, especially with respect to determining the acceptable risk. The road to a cure is going to be difficult, but it is vital that inevitable disappointments do not detract from the final goal, which remains worth striving for.

Effect of AIDS on women who have sex-determined health issues

Worldwide, women account for a growing percentage of human immunodeficiency virus (HIV)-infected patients and more than half of all HIV infections. For many years, morphologic imaging methods were the main approaches employed to investigate HIV and its complications. However, during the past decade, advancements in PET and SPECT imaging technologies opened new possibilities for improved understanding of the pathophysiological processes in HIV. Diagnosis of early HIV-associated neurocognitive disorders (HAND) is important, as many of its symptoms can be caused by other conditions common to people with HIV/AIDS. Presently, there are no PET and SPECT tracers or combination of markers for HAND, hence novel HAND-specific tracers are needed if nuclear medicine is to play a role in solving the problem of the HAND "epidemic." As both highly active antiretroviral therapy (HAART)-induced lipoatrophy and cardiovascular diseases are characterized by ongoing inflammation, FDG-PET/CT imaging may represent an important imaging technique for better understanding the metabolic risk in HIV-infected women on HAART. HIV-infected women are at increased risk for the development of human papilloma virus-associated neoplasms such as cervical and anal carcinomas; these aggressive tumors could be treated better with integration of FDG-PET as part of the standard pretreatment workup. A similar value of FDG-PET may be realized in women with HIV-associated Kaposi sarcoma, as they have more extensive cutaneous disease than men do. In the era of HAART, the incidence and local invasiveness of breast cancer may change, thus creating a need to redefine the pathophysiology of breast cancer in HIV-positive women. Finally, mammary tuberculosis, occasionally the presenting symptom in HIV-infected women, may present with nonspecific clinical, radiological, and histologic findings. In these women, FDG-PET can be of value to detect the lesion for a representative biopsy, staging to exclude pulmonary and other extrapulmonary lesions, and also for therapy monitoring.

Dispelling myths and focusing on notable concepts in HIV pathogenesis

Since the discovery of HIV over three decades ago, major efforts have been made to control and perhaps eliminate HIV infection worldwide. During these studies, certain myths or misconceptions about this infectious disease have been emphasized and other potentially beneficial concepts have received less attention. A true long-term solution to HIV infection merits an appreciation of alternative ideas and findings that could be beneficial in the ultimate control of HIV/AIDS. Here, I discuss six issues and call for more attention to the science of HIV and well-designed clinical trials.

Whole-body immunoPET reveals active SIV dynamics in viremic and antiretroviral therapy-treated macaques

The detection of viral dynamics and localization in the context of controlled HIV infection remains a challenge and is limited to blood and biopsies. We developed a method to capture total-body simian immunodeficiency virus (SIV) replication using immunoPET (antibody-targeted positron emission tomography). The administration of a poly(ethylene glycol)-modified, (64)Cu-labeled SIV Gp120-specific antibody led to readily detectable signals in the gastrointestinal and respiratory tract, lymphoid tissues and reproductive organs of viremic monkeys. Viral signals were reduced in aviremic antiretroviral-treated monkeys but detectable in colon, select lymph nodes, small bowel, nasal turbinates, the genital tract and lung. In elite controllers, virus was detected primarily in foci in the small bowel, select lymphoid areas and the male reproductive tract, as confirmed by quantitative reverse-transcription PCR (qRT-PCR) and immunohistochemistry. This real-time, in vivo viral imaging method has broad applications to the study of immunodeficiency virus pathogenesis, drug and vaccine development, and the potential for clinical translation.

The potential of radioimmunotherapy as a new hope for HIV patients

HIV/AIDS remains an enormous public health burden. Advances in anti-retroviral therapy (ART) have greatly reduced mortality and morbidity but HIV remains incurable, with patients suffering numerous disease- and treatment-related side effects. Any curative strategy for HIV must selectively eliminate existing infected cells. Radioimmunotherapy (RIT) is an established clinical modality in cancer treatment and has been shown to be effective in multiple infectious diseases models. We have recently demonstrated that RIT using a gp41-targeting antibody was effective and safe in eliminating HIV-infected cells in vivo (in mice), in vitro, and ex vivo in cells from HIV patients treated with ART. In addition, there is strong evidence that this radiolabeled antibody can eliminate HIV infected cells across the blood brain barrier. We consider RIT to be the most promising backbone strategy for HIV eradication.

Eradicating HIV-1 infection: seeking to clear a persistent pathogen

Effective antiretroviral therapy (ART) blunts viraemia, which enables HIV-1-infected individuals to control infection and live long, productive lives. However, HIV-1 infection remains incurable owing to the persistence of a viral reservoir that harbours integrated provirus within host cellular DNA. This latent infection is unaffected by ART and hidden from the immune system. Recent studies have focused on the development of therapies to disrupt latency. These efforts unmasked residual viral genomes and highlighted the need to enable the clearance of latently infected cells, perhaps via old and new strategies that improve the HIV-1-specific immune response. In this Review, we explore new approaches to eradicate established HIV-1 infection and avoid the burden of lifelong ART.

Targeting strategies for delivery of anti-HIV drugs

Human Immunodeficiency Virus (HIV) infection remains a significant cause of mortality globally. Though antiretroviral therapy has significantly reduced AIDS-related morbidity and mortality, there are several drawbacks in the current therapy, including toxicity, drug–drug interactions, development of drug resistance, necessity for long-term drug therapy, poor bio-availability and lack of access to tissues and reservoirs. To circumvent these problems, recent anti-HIV therapeutic research has focused on improving drug delivery systems through drug delivery targeted specifically to host cells infected with HIV or could potentially get infected with HIV. In this regard, several surface molecules of both viral and host cell origin have been described in recent years, that would enable targeted drug delivery in HIV infection. In the present review, we provide a comprehensive overview of the need for novel drug delivery systems, and the successes and challenges in the identification of novel viral and host-cell molecules for the targeted drug delivery of anti-HIV drugs. Such targeted anti-retroviral drug delivery approaches could pave the way for effective treatment and eradication of HIV from the body.

Breaking the mould - novel diagnostic and therapeutic strategies for invasive pulmonary aspergillosis in the immune deficient patient

Invasive pulmonary aspergillosis (IPA) caused by the ubiquitous environmental fungus Aspergillus is a frequently fatal lung disease of immunocompromised humans accounting for more than 200,000 infections each year, with an associated mortality rate of 30-90%. This review addresses the current status of IPA diagnosis and treatment and the urgent need to develop accurate, non-invasive strategies for identifying pulmonary infections in the ever-expanding population of immune deficient patients at risk of acquiring opportunistic fungal infections including hematological malignancy and hematopoetic stem cell transplant patients. Recent advances in the use of an Aspergillus-specific monoclonal antibody, JF5, for point-of-care diagnosis of IPA using lateral-flow technology is examined, as is its use in PET/MRI bioimaging and radio-immunotherapy using radionuclide-labeled single chain antibody fragments, Fab fragments, and a fully humanized JF5 derivative.

Radioimmunotherapy with α-particle-emitting radionuclides

α-particle-emitting radionuclides are highly cytotoxic and are thus promising candidates for use in targeted radioimmunotherapy of cancer. Due to their high linear energy transfer (LET) combined with a short path length in tissue, α-particles cause severe DNA double-strand breaks that are repaired inaccurately and finally trigger cell death. For radioimmunotherapy, α-emitters such as 225Ac, 211At, 212Bi/212Pb, 213Bi and 227Th are coupled to antibodies via appropriate chelating agents. The α-emitter immunoconjugates preferably target proteins that are overexpressed or exclusively expressed on cancer cells. Application of α-emitter immunoconjugates seems particularly promising in treatment of disseminated cancer cells and small tumor cell clusters that are released during the resection of a primary tumor. α-emitter immunoconjugates have been successfully administered in numerous experimental studies for therapy of ovarian, colon, gastric, blood, breast and bladder cancer. Initial clinical trials evaluating α-emitter immunoconjugates in terms of toxicity and therapeutic efficacy have also shown positive results in patients with melanoma, ovarian cancer, acute myeloid lymphoma and glioma. The present problems in terms of availability of therapeutically effiective α-emitters will presumably be solved by use of alternative production routes and installation of additional production facilities in the near future. Therefore, clinical establishment of targeted α-emitter radioimmunotherapy as one part of a multimodal concept for therapy of cancer is a promising, middle-term concept.

Targeted alpha-therapy using [Bi-213]anti-CD20 as novel treatment option for radio- and chemoresistant non-Hodgkin lymphoma cells

Radioimmunotherapy (RIT) is an emerging treatment option for non-Hodgkin lymphoma (NHL) producing higher overall response and complete remission rates compared with unlabelled antibodies. However, the majority of patients treated with conventional or myeloablative doses of radiolabelled antibodies relapse. The development of RIT with alpha-emitters is attractive for a variety of cancers because of the high linear energy transfer (LET) and short path length of alpha-radiation in human tissue, allowing higher tumour cell kill and lower toxicity to healthy tissues. In this study, we investigated the molecular effects of the alpha-emitter Bi-213 labelled to anti-CD20 antibodies ([Bi-213]anti-CD20) on cell cycle and cell death in sensitive and radio-/chemoresistant NHL cells. [Bi-213]anti-CD20 induced apoptosis, activated caspase-3, caspase-2 and caspase-9 and cleaved PARP specifically in CD20-expressing sensitive as well as in chemoresistant, beta-radiation resistant and gamma-radiation resistant NHL cells. CD20 negative cells were not affected by [Bi-213]anti-CD20 and unspecific antibodies labelled with Bi-213 could not kill NHL cells. Breaking radio-/chemoresistance in NHL cells using [Bi-213]anti-CD20 depends on caspase activation as demonstrated by complete inhibition of [Bi-213]anti-CD20-induced apoptosis with zVAD.fmk, a specific inhibitor of caspases activation. This suggests that deficient activation of caspases was reversed in radioresistant NHL cells using [Bi-213]anti-CD20. Activation of mitochondria, resulting in caspase-9 activation was restored and downregulation of Bcl-x_L and XIAP, death-inhibiting proteins, was found after [Bi-213]anti-CD20 treatment in radio-/chemosensitive and radio-/chemoresistant NHL cells. [Bi-213]anti-CD20 seems to be a promising radioimmunoconjugate to improve therapeutic success by breaking radio- and chemoresistance selectively in CD20-expressing NHL cells via re-activating apoptotic pathways through reversing deficient activation of caspases and the mitochondrial pathway and downregulation of XIAP and Bcl-x_L.

Systemic targeted alpha radiotherapy for cancer

BACKGROUND

The fundamental principles of internal targeted alpha therapy forcancer were established many decades ago.The high linear energy transfer (LET) ofalpha radiation to the targeted cancer cellscauses double strand breaks in DNA. Atthe same time, the short range radiation spares adjacent normal tissues. This targeted approach complements conventional external beam radiotherapy and chemotherapy. Such therapies fail on several fronts, such as lack of control of some primary cancers (e.g. glioblastoma multiforme) and to inhibit the development of lethal metastaticcancer after successful treatment of the primary cancer.

OBJECTIVE

This review charts the developing role of systemic high LET, internalradiation therapy.

METHOD

Targeted alpha therapy is a rapidly advancing experimental therapy thatholds promise to deliver high cytotoxicity to targeted cancer cells. Initially thoughtto be indicated for leukemia and micrometastases, there is now evidence that solidtumors can also be regressed.

RESULTS

Alpha therapy may be molecular or physiological in its targeting. Alphaemitting radioisotopes such as Bi-212, Bi-213, At-211 and Ac-225 are used to labelmonoclonal antibodies or proteins that target specific cancer cells. Alternatively, Radium-233 is used for palliative therapy of breast and prostate cancers because of its bone seeking properties.

CONCLUSION

Preclinical studies and clinical trials of alpha therapy are discussedfor leukemia, lymphoma, melanoma, glioblastoma multiforme, bone metastases, ovarian cancer, pancreatic cancer and other cancers.

Curing HIV: lessons from cancer therapy

PURPOSE OF REVIEW

Interest in finding a potential 'cure' for HIV has taken on greater interest and urgency since the report of an individual who underwent allogeneic stem cell transplant from a CCR5 delta 32 homozygote donor after high-dose chemotherapy for acute myeloid leukemia. The potential role of cancer chemotherapy and other cancer-directed treatment approaches is discussed in the context of their potential role in helping to eliminate HIV from the infected host.

RECENT FINDINGS

Cancer chemotherapy and other cancer-targeted agents have been used successfully in treating a variety of malignancies in both HIV-infected and HIV-uninfected individuals. Lessons learned from these strategies may be of importance in helping to define more effective ways of controlling and eliminating HIV as well. Application of these anticancer strategies to patients with HIV are beginning to be explored and may help determine their potential usefulness in this disease as well.

SUMMARY

Although cytotoxic chemotherapy is a crude and not particularly effective way of removing HIV latently infected cells and tissue reservoirs, several new approaches to targeting and controlling cancer proliferation may be of value in HIV cure research and may one day help to end this disease.

A novel class of anti-HIV agents with multiple copies of enfuvirtide enhances inhibition of viral replication and cellular transmission in vitro

We constructed novel HIV-1 fusion inhibitors that may overcome the current limitations of enfuvirtide, the first such therapeutic in this class. The three prototypes generated by the Dock-and-Lock (DNL) technology to comprise four copies of enfuvirtide tethered site-specifically to the Fc end of different humanized monoclonal antibodies potently neutralize primary isolates (both R5-tropic and X4-tropic), as well as T-cell-adapted strains of HIV-1 in vitro. All three prototypes show EC₅₀ values in the subnanomolar range, which are 10- to 100-fold lower than enfuvirtide and attainable whether or not the constitutive antibody targets HIV-1. The potential of such conjugates to purge latently infected cells was also demonstrated in a cell-to-cell viral inhibition assay by measuring their efficacy to inhibit the spread of HIV-1_LAI from infected human peripheral blood mononuclear cells to Jurkat T cells over a period of 30 days following viral activation with 100 nM SAHA (suberoylanilide hydroxamic acid). The IgG-like half-life was not significantly different from that of the parental antibody, as shown by the mean serum concentration of one prototype in mice at 72 h. These encouraging results provide a rationale to develop further novel anti-HIV agents by coupling additional antibodies of interest with alternative HIV-inhibitors via recombinantly-produced, self-assembling, modules.

Acinetobacter baumannii: an emerging opportunistic pathogen

Acinetobacter baumannii is an opportunistic bacterial pathogen primarily associated with hospital-acquired infections. The recent increase in incidence, largely associated with infected combat troops returning from conflict zones, coupled with a dramatic increase in the incidence of multidrug-resistant (MDR) strains, has significantly raised the profile of this emerging opportunistic pathogen. Herein, we provide an overview of the pathogen, discuss some of the major factors that have led to its clinical prominence and outline some of the novel therapeutic strategies currently in development.

Pre-clinical evaluation of a 213Bi-labeled 2556 antibody to HIV-1 gp41 glycoprotein in HIV-1 mouse models as a reagent for HIV eradication

Background

Any strategy for curing HIV infection must include a method to eliminate viral-infected cells. Based on our earlier proof-of-principle results targeting HIV-1 infected cells with radiolabeled antibody (mAb) to gp41 viral antigen, we embarked on identifying a suitable candidate mAb for preclinical development.

Methodology/Principal Findings

Among the several human mAbs to gp41 tested, mAb 2556 was found to have high affinity, reactivity with multimeric forms of gp41 present on both the surface of virus particles and cells expressing HIV-1 Env, and recognition of a highly conserved epitope of gp41 shared by all HIV-1 subtypes. Also, mAb 2556 was the best in competition with HIV-1+ serum antibodies, which is an extremely important consideration for efficacy in the treatment of HIV patients. When radiolabeled with alpha-emitting radionuclide 213-Bismuth (²¹³Bi) - ²¹³Bi-2556 efficiently and specifically killed ACH-2 human lymphocytes chronically infected with HIV-1, and HIV-1 infected human peripheral blood mononuclear cells (hPBMCs). The number of binding sites for ²¹³Bi-2556 on the surface of the infected cells was >10⁶. The in vivo experiments were performed in two HIV-1 mouse models – splenic and intraperitoneal. In both models, the decrease in HIV-1 infected hPBMCs from the spleens and peritoneum, respectively, was dose-dependent with the most pronounced killing of hPBMCs observed in the 100 µCi ²¹³Bi-2556 group (P = 0.01). Measurement of the blood platelet counts and gross pathology of the treated mice demonstrated the lack of toxicity for ²¹³Bi-2556.

Conclusions/Significance

We describe the preclinical development of a novel radiolabeled mAb reagent that could potentially be part of an HIV eradication strategy that is ready for translation into the clinic as the next step in its development. As viral antigens are very different from “self” human antigens - this approach promises high selectivity, increased efficacy and low toxicity, especially in comparison to immunotoxins.

Kinase control prevents HIV-1 reactivation in spite of high levels of induced NF-κB activity

Despite its clinical importance, the molecular biology of HIV-1 latency control is at best partially understood, and the literature remains conflicting. The most recent description that latent HIV-1 is integrated into actively expressed host genes has further confounded the situation. This lack of molecular understanding complicates our efforts to identify therapeutic compounds or strategies that could reactivate latent HIV-1 infection in patients, a prerequisite for the eradication of HIV-1 infection. Currently, many therapeutic development efforts operate under the assumption that a restrictive histone code could govern latent infection and that either dissipation of the histone-based restrictions or NF-κB activation could be sufficient to trigger HIV-1 reactivation. We here present data that suggest an additional, higher level of molecular control. During a high-content drug screening effort, we identified AS601245 as a potent inhibitor of HIV-1 reactivation in latently infected primary T cells and T cell lines. In either system, AS601245 inhibited HIV-1 reactivation despite high levels of induced NF-κB activation. This finding suggests the presence of a gatekeeper kinase activity that controls latent HIV-1 infection even in the presence of high levels of NF-κB activity. Potential therapeutic stimuli that do not target this gatekeeper kinase will likely fail to trigger efficient system-wide HIV-1 reactivation.

Radioimmunotherapy of fungal diseases: the therapeutic potential of cytocidal radiation delivered by antibody targeting fungal cell surface antigens

Radioimmunotherapy is the targeted delivery of cytocidal radiation to cells via specific antibody. Although mature for the treatment of cancer, RIT of infectious diseases is in pre-clinical development. However, as there is an obvious and urgent need for novel approaches to treat infectious diseases, RIT can provide us with a powerful approach to combat serious diseases, including invasive fungal infections. For example, RIT has proven more effective than standard amphotericin B for the treatment of experimental cryptococcosis. This review will discuss the concepts of RIT, its applications for infectious diseases, and the strides made to date to bring RIT of infectious diseases to fruition. Finally, we will discuss the potential of PAN-FUNGAL RIT, the targeting of conserved fungal cell surface antigens by RIT, as a treatment modality for fungi prior to the formal microbiological identification of the specific pathogen. In sum, RIT provides a mechanism for the targeted killing of drug susceptible or resistant fungi irrespective of the host immune status and may dramatically reduce the length of therapy currently required for many invasive fungal diseases.

Cryptococcus neoformans as a Model for Radioimmunotherapy of Infections

There is an obvious and urgent need for novel approaches to treat infectious diseases. The use of monoclonal antibodies in therapy of infectious diseases is now experiencing renewed interest. During the last 5 years radioimmunotherapy (RIT), a modality previously developed only for cancer treatment, has been successfully adapted for the treatment of experimental fungal, bacterial, and viral infections. As our model organism for studying the efficacy, mechanisms, potential toxicity, and radioresistance to RIT, as well as for comparison of RIT with the existing antimicrobial therapies we have chosen the encapsulated yeast Cryptococcus neoformans (CN). The success of RIT approach in laboratory studies provides encouragement for feasibility of therapeutically targeting microbes with labeled antibodies. In addition, the creation of “panantibodies” for RIT which would recognize antigens shared by the whole class of pathogens such as fungi, for example, would facilitate the introduction of RIT into the clinic.

Targeted cytotoxic therapy: adapting a rapidly progressing anticancer paradigm for depletion of persistent HIV-infected cell reservoirs

PURPOSE OF REVIEW

HIV-infected cells persisting in the face of highly active antiretroviral therapy are arguably the greatest hurdle to eradication of the virus from the body. Complementary strategies aimed at selective killing of infected cells are described.

RECENT FINDINGS

Pioneered by research in the cancer field, various approaches are under development for selective killing of HIV-infected cells. These include targeted cytotoxic proteins, adoptive cell therapy, cytocidal virotherapy, and targeted nonbiological drug carriers.

SUMMARY

These developmental efforts may provide a critical complement to antiretroviral therapy in efforts to achieve HIV eradication, or a 'functional cure' whereby therapy can be stopped without viral rebound.

Radiolabeled antiviral drugs and antibodies as virus-specific imaging probes

A number of small-molecule drugs inhibit viral replication by binding directly to virion structural proteins or to the active site of a viral enzyme, or are chemically modified by a viral enzyme before inhibiting a downstream process. Similarly, antibodies used to prevent or treat viral infections attach to epitopes on virions or on viral proteins expressed on the surface of infected cells. Such drugs and antibodies can therefore be thought of as probes for the detection of viral infections, suggesting that they might be used as radiolabeled tracers to visualize sites of viral replication by single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. A current example of this approach is the PET imaging of herpes simplex virus infections, in which the viral thymidine kinase phosphorylates radiolabeled thymidine analogues, trapping them within infected cells. One of many possible future applications might be the use of a radiolabeled hepatitis C protease inhibitor to image infection in animals or humans and provide a quantitative measure of viral burden. This article reviews the basic features of radionuclide imaging and the characteristics of ideal tracer molecules, and discusses how antiviral drugs and antibodies could be evaluated for their suitability as virus-specific imaging probes. The use of labeled drugs as low-dose tracers would provide an alternative application for compounds that have failed to advance to clinical use because of insufficient in vivo potency, an unsuitable pharmacokinetic profile or hepato- or nephrotoxicity.

Immunotherapy of viral infections

Among the microorganisms that cause diseases of medical or veterinary importance, the only group that is entirely dependent on the host, and hence not easily amenable to therapy via pharmaceuticals, is the viruses. Since viruses are obligate intracellular pathogens, and therefore depend a great deal on cellular processes, direct therapy of viral infections is difficult. Thus, modifying or targeting nonspecific or specific immune responses is an important aspect of intervention of ongoing viral infections. However, as a result of the unavailability of effective vaccines and the extended duration of manifestation, chronic viral infections are the most suitable for immunotherapies. We present an overview of various immunological strategies that have been applied for treating viral infections after exposure to the infectious agent.

Inhibition of in vivo HIV infection in humanized mice by gene therapy of human hematopoietic stem cells with a lentiviral vector encoding a broadly neutralizing anti-HIV antibody

Due to the inherent immune evasion properties of the HIV envelope, broadly neutralizing HIV-specific antibodies capable of suppressing HIV infection are rarely produced by infected individuals. We examined the feasibility of utilizing genetic engineering to circumvent the restricted capacity of individuals to endogenously produce broadly neutralizing HIV-specific antibodies. We constructed a single lentiviral vector that encoded the heavy and light chains of 2G12, a broadly neutralizing anti-HIV human antibody, and that efficiently transduced and directed primary human B cells to secrete 2G12. To evaluate the capacity of this approach to provide protection from in vivo HIV infection, we used the humanized NOD/SCID/gamma(c)(null) mouse model, which becomes populated with human B cells, T cells, and macrophages after transplantation with human hematopoietic stem cells (hu-HSC) and develops in vivo infection after inoculation with HIV. The plasma of the irradiated NOD/SCID/gamma(c)(null) mice transplanted with hu-HSC transduced with the 2G12-encoding lentivirus contained 2G12 antibody, likely secreted by progeny human lymphoid and/or myeloid cells. After intraperitoneal inoculation with high-titer HIV-1(JR-CSF), mice engrafted with 2G12-transduced hu-HSC displayed marked inhibition of in vivo HIV infection as manifested by a profound 70-fold reduction in plasma HIV RNA levels and an almost 200-fold reduction in HIV-infected human cell numbers in mouse spleens, compared to control hu-HSC-transplanted NOD/SCID/gamma(c)(null) mice inoculated with equivalent high-titer HIV-1(JR-CSF). These results support the potential efficacy of this new gene therapy approach of using lentiviral vectors encoding a mixture of broadly neutralizing HIV antibodies for the treatment of HIV infection, particularly infection with multiple-drug-resistant isolates.

Monoclonal antibody-based therapies for microbial diseases

The monoclonal antibody (mAb) revolution that currently provides many new options for the treatment of neoplastic and inflammatory diseases has largely bypassed the field of infectious diseases. Only one mAb is licensed for use against an infectious disease, although there are many in various stages of development. This situation is peculiar given that serum therapy was one of the first effective treatments for microbial diseases and that specific antibodies have numerous antimicrobial properties. The underdevelopment and underutilization of mAb therapies for microbial diseases has various complex explanations that include the current availability of antimicrobial drugs, small markets, high costs and microbial antigenic variation. However, there are signs that the climate for mAb therapeutics in infectious diseases is changing given increasing antibiotic drug resistance, the emergence of new pathogenic microbes for which no therapy is available, and development of mAb cocktail formulations. Currently, the major hurdle for the widespread introduction of mAb therapies for microbial diseases is economic, given the high costs of immunoglobulin preparations and relatively small markets. Despite these obstacles there are numerous opportunities for mAb development against microbial diseases and the development of radioimmunotherapy provides new options for enhancing the magic bullet. Hence, there is cautious optimism that the years ahead will see more mAbs in clinical use against microbial diseases.

Radiolabeled antibodies to Bacillus anthracis toxins are bactericidal and partially therapeutic in experimental murine anthrax

Bacillus anthracis is a powerful agent for use in biological warfare, and infection with the organism is associated with a high rate of mortality, underscoring the need for additional effective therapies for anthrax. Radioimmunotherapy (RIT) takes advantage of the specificity and affinity of the antigen-antibody interaction to deliver a microbicidal radioactive nuclide to a site of infection. RIT has proven therapeutic in experimental models of viral, bacterial, and fungal infections; but it is not known whether this approach can successfully employ toxin binding monoclonal antibodies (MAbs) for diseases caused by toxigenic bacteria. Indirect immunofluorescence studies with MAbs to protective antigen (MAbs 7.5G gamma2b and 10F4 gamma1) and lethal factor (MAb 14FA gamma2b) revealed the surface expression of toxins on bacterial cells. Scatchard analysis of MAbs revealed high binding constants and numerous binding sites on the bacterial surface. To investigate the microbicidal properties of these MAbs, our group radiolabeled MAbs with either (188)Re or (213)Bi. In vitro, (213)Bi was more efficient than (188)Re in mediating microbicidal activity against B. anthracis. The administration of MAbs [(213)Bi]10F4 gamma1 and [(213)Bi]14FA gamma2b prolonged the survival of A/JCr mice infected with B. anthracis Sterne bacterial cells but not B. anthracis Sterne spores. These results indicate that RIT with MAbs that target B. anthracis toxin components can be used to treat experimental anthrax infection and suggest that toxigenic bacteria may be targeted with radiolabeled MAbs.