Some serum protein nitrogen mustard complexes with high chemotherapeutic selectivity

Albumin-drug conjugates in the treatment of hepatic disorders

INTRODUCTION

This review deals with the use of serum albumin (SA) as a carrier for the selective delivery of drugs to liver cells.

AREAS COVERED

The synthesis and properties of the SA conjugates prepared to enhance the performance of the drugs used in the treatment of viral hepatitis, hepatocellular carcinoma (HCC), liver micrometastases and hepatic fibrosis are reported.

EXPERT OPINION

Studies in humans and laboratory animals demonstrated the capacity of SA conjugates to accomplish a liver targeting of the drugs, but at the same time underscored their limits and drawbacks, which can explain why to date these complexes did not reach a practical application. The major drawback is the need of administration by intravenous route, which prevents long-term daily treatments as required by some liver pathologies, such as chronic virus hepatitis and fibrosis. At present, only a conjugate carrying doxorubicin and addressed to the treatment of HCC showed in laboratory animals a solid potentiality to improve the value of the coupled drug. In the future, conjugation to SA could remain a successful strategy to permit the administration of drugs with rapid resolutive effects inside liver cells without causing severe extrahepatic adverse reactions.

Nanotechnology and site-targeted drug delivery

Nanotechnology, building on its ability to control or manipulate structures at the atomic level, promises to develop effective drug-delivery systems. This is to be achieved through creating structures that have novel properties because of their small size. This is not an entirely new concept in site-targeted drug delivery, and this critical review examines recent contributions made by 'nanotechnology' to solve critical issues concerning the development of therapeutically effective and acceptable site-targeted drug delivery systems. It is shown that very little progress has been made. For nanotechnology rationally to generate materials useful in human therapy it will need to progress in full recognition of all the requirements biology places on the acceptability of exogenous materials.

Essential properties of drug-targeting delivery systems

How, if at all, can drug delivery help to create ideal drugs? After four decades of trying, an effective site-specific drug-delivery system has not yet been developed. This review draws attention to the pharmacokinetic conditions that must be met to achieve a successful performance by site-selective drug-carrier delivery systems. In a drug-carrier approach, a drug is attached to a macromolecular carrier via a chemically labile linker. The carrier transports the drug to its site of action and releases it at the target site. For this simple approach to work, several fundamental conditions (nonspecific interactions, target site access, drug release and drug suitability) must be satisfied. The importance of these essential requirements, not always recognized in the development of drug-delivery systems, is discussed and illustrated by recent examples selected from the literature.

Targeting of drugs to cell surface receptors

The new approach to the treatment of cancer or to immunomodulation is drug targeting. The effort to achieve either an absolute or a relative amplification of the tumoricidal effect of anticancer drugs through increased generation or acquisition of reactive molecules at the tumor site or a reduction of the toxic molecules available to the periphery has led to a number of strategies. Among them are (1) targeting using antibodies to their fragments, hormones, carbohydrates, and growth factors; (2) retargeting using bispecific antibodies; (3) construction of chimeric genes; (4) streptavidin-biotin based immunotherapy; (5) prodrug activation strategies (ADEPT); (6) antibody-targeted superantigens; and (7) gene delivery for the purpose of gene therapy.

A novel covalent modification of antibodies at their amino groups with retention of antigen-binding activity

A novel method of covalent modification of antibodies at their amino groups with retention of antigen-binding activity is described. The procedure is as follows: (a) blockade of those amino groups of antibodies whose integrity is essential to their antigen-binding activity with 2,3-dimethylmaleic anhydride, a reversible amino group-blocking reagent; (b) modification of residual amino groups with reagents reactive with the amino groups; and (c) removal of dimethylmaleyl groups by hydrolysis. This procedure was used for covalent conjugation of methotrexate (MTX) with two monoclonal antibodies against human melanoma-associated antigens using MTX N-succinimidyl ester. MTX attached to the antibodies at sites other than the amino groups via less stable bond(s) was removed by treatment with hydroxylamine.

Mitomycin C carrying microspheres as a novel method of drug delivery

Biodegradable albumin microspheres containing about 5% mitomycin C (MMC) were prepared in an average diameter of 45 +/- 8 microns by heat denaturation in oil at 120 degrees C and/or cross-linking with glutaraldehyde. These MMC microspheres released, in vitro, about 20% of the contained MMC for over 3 days, and they were intra-arterially infused into albino rabbits and Wistar rats, as a preclinical model of intra-arterial infusion treatment for patients with inoperable hepatic tumor. We infused these microspheres into the femoral artery of rabbits with a VX-2 tumor implanted into the flank of the hindleg. High levels of MMC were maintained for several hours in the tumor and the entrapped MMC microspheres were detected within arterioles in the VX-2 tumors. The growth of VX-2 tumor was inhibited considerably, compared to findings in the control rabbits given conventional MMC. In the next studies, MMC microspheres were infused into the rat hepatic artery, and the levels of MMC in the hepatic vein blood were maintained at much the same concentration for over 2 hours after the infusion, in marked contrast to rapid decreases in the conventional MMC. Histologic findings revealed that MMC micro-spheres were entrapped within the hepatic arterioles for over 2 weeks and released biologically active MMC into the neighboring tissues for prolonged periods of time.

Targeting potential of antibody conjugates

Although the use of conjugates of enzymes has been considered, their use has not been very actively pursued. Much more interest has focused on the possibilities offered by the use of toxins, their subunits or of ribosome inhibitors. Conjugates of holotoxin which were very active and specific in vitro have been prepared. High in vivo activity and some specificity together with reduced whole body toxicity has been described. When A chain subunits or ribosome inhibiting proteins are used, the results are more mixed. Some very active and specific conjugates are known but others have relatively low activity. The reasons for this may be associated with the particular antigen to which the antibody component is directed, the nature of the A chain or inhibitor and the type and physiological state of the target cell. Application to man seems likely in the first instance to involve the removal of undesirable cells from bone marrow transplants.