Immunotoxin

WHO (2004) concluded that The mono- and disubstituted compounds that may leach from PVC water pipes for a short time after installation are primarily immunotoxins although they appear to be of low general toxicity, some are developmental toxins in rodents. The data available are insufficient to permit the proposal of guideline values for individual dialkyltins or the mono derivatives, although the concentrations observed in drinking-water are several orders of magnitude lower than the doses reported to cause developmental effects in rats and mice. ... 

Wiley, R. G., Oeltmann, T. N. Lappi, D. A. (1991). Immunolesioning selective destruction of neurons using immunotoxin to rat NGF receptor. Brain Res. 562... 

Krum JM, Kenyon KL, Rosenstein JM. Expression of blood-brain barrier characteristics following neuronal loss and astroglial damage after administration of anti-thy-1 immunotoxin. Exp Neurol 1997 146 33-45. 

SMPT, succinimidyloxycarbonyl-a-methyl-a-(2-pyridyldithio)toluene, contains an NHS ester end and a pyridyl disulfide end similar to SPDP, but its hindered disulfide makes conjugates formed with this reagent more stable (Thorpe et al., 1987) (Chapter 5, Section 1.2). The reagent is especially useful in forming immunotoxin conjugates for in vivo administration (Chapter 21, Section 2.1). A water-soluble analog of this crosslinker containing an extended spacer arm is also commercially available as sulfo-LC-SMPT (Thermo Fisher). 

SMPT or sulfo-LC-SMPT has been used to develop conjugates for in vivo delivery of siRNA to hepatocytes (Rozema et al., 2007), in preparing an anti-CD25-immunotoxin conjugate (Mielke et al., 2007), and in preparing conjugates for selective depletion of donor lymphocytes in stem cell transplantation (Solomon et al., 2005). 

PDPH has been used in the preparation of immunotoxin conjugates (Zara et al., 1991). It has also been used to create a unique conjugate of nerve growth factor (NGF) with an... 

Figure 21.1 The basic design of an immunotoxin conjugate consists of an antibody-targeting component crosslinked to a toxin molecule. The complexation typically includes a disulfide bond between the antibody portion and the cytotoxic component of the conjugate to allow release of the toxin intracellularly. In this illustration, an intact A-B toxin protein provides the requisite disulfide, but the linkage also may be designed into the crosslinker itself.

Since immunotoxin conjugates are destined to be used in vivo, their preparation involves more critical consideration of crosslinking methods than most of the other conjugation protocols described in this book. The following sections discuss the problems associated with toxin conjugates and the main crosslinking methods for preparing them. 

It has become apparent that the method of crosslinking can dramatically affect the activity of an immunotoxin in vivo. This is true not only with regard to possible direct blocking by the crosslinker of the enzymatic active site which is responsible for inactivation of ribosomes, but the chemistry of conjugation also is an important factor in proper binding and entry of the... 

A-chain immunotoxins, however, may not be quite as cytotoxic as conjugates formed from intact toxin molecules (Manske et al., 1989). In an alternative approach to A chain use, the intact toxin of two-subunit proteins is directly conjugated to a monoclonal without isolation of the A chain. Conjugation of an antibody with intact A-B chain toxins can be done without a cleavable linker, as long as the A chain can still separate from the B chain once it is internalized. Therefore, it is important to avoid intramolecular crosslinking during the conjugation process which can prevent release of the A-B complex. In addition, since the B chain... 

Regardless of their method of preparation, the required and ideal characteristics of immunotoxin conjugates can be summarized in the following points ... 

Administration of the immunotoxin should result in cell death and complete elimination of all target cells. 

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