ODRUG classes

Alkylating Agents

The alkylating agents are a class of drugs that are capable of forming covalent bonds with important biomolecules. The major targets of drug action are nucleophilic groups present on DNA (especially the 7-position of guanine); however, proteins and RNA among others may also be alkylated. Alkylation of DNA is thought to lead to cell death, although the exact mechanism is uncertain. Potential mechanisms of cell death include activation of apoptosis caused by p53 activation and disruption of the template function of DNA. In many cases, however, the cancer cells have dysfunctional p53 so that even though the cell has been unable to replicate DNA error free, cell death via apoptosis does not occur. In this way, cancer cells may become resistant to the effects of alkylating agents. Another possibility is that the cancer cells, like normal cells, have various mechanisms by which alkylated DNA bases can be excised.

Disruption of the template function of DNA may have several effects. There are several potential nucleophilic sites on DNA, which are susceptible to electrophilic attack by an al-kylating agent (N-2, N-3, and N-7 of guanine, N-1, N-3, and N-7 of adenine, 0-6 of thymine, N-3 of cytosine). The most important of these for many alkylating agents is the N-7 position of guanine whose nucleophilicity may be enhanced by adjacent guanine residues. Alkylation converts the base to an effective leaving group so that attack by water leads to depuri-nation and the loss of genetic information if the resulting depurination is not repaired by the cell (Scheme 10.1). Additionally, alkylation has been proposed to result in altered base pairing away from the normal G-C: A-T hydrogen bonds because of alterations in tautomerization.4 The alkylation also leads to increased acidity of the N-1 nitrogen reducing the pKa from 9 to 7 to 8 giving rise to a zwitterionic form that may also mispair. For those agents that possess two reactive functionalities, both interstrand and intrastrand cross-linking becomes possible. When interstrand links occur, separation of the two strands during replication is prevented and therefore replication is blocked.

Most of the currently used alkylating agents are nonselective regarding the sequence of DNA with which they react. Therefore, it is uncertain whether alkylation will lead to a cytotoxic event. Some alkylation reactions may lead to inconsequential results, and others may be easily repaired by the cell. Recent efforts have been directed at producing more sequence-selective alkylators that could be used at lower concentrations in an effort to reduce the significant side effects associated with this group of agents.

The general mechanism for alkylation involves nucleo-philic attack by —N=, —NH2, —OH, —O—PO3H of DNA and RNA, while additional nucleophiles (—SH, COOH, etc.) present on proteins may also react (Scheme 10.2). Anion

Adenine Derivatives
Scheme 10.1 • Alkylation of guanine N-7 and subsequent depurination of DNA.

Scheme 10.2 • General reaction for alkylation and inactivation of alkylating agents.

formation increases the reactivity of the nucleophile compared with the un-ionized form (—is more nucleophilic than OH). Reaction with water is also possible, because it represents the nucleophile in greatest abundance in the body and this becomes more likely as the electrophile becomes more reactive. Reaction involves displacement of a leaving group on the electrophile by the nucleophile. The reactivity of the electrophile is dependent in part on the ability of the leaving group to stabilize a negative charge.

Along with a common mechanism, there are other characteristics that the alkylating agents share. Mechanisms by which cells may become resistant to these agents are thought to be similar and include decreased cellular uptake, increased inactivation by detoxifying nucleophilic thiols such as glutathione, increased DNA repair processes, and decreased drug activation when this is necessary for generation of an alkylat-ing species. Functioning apoptotic mechanisms are thought to be important for the effectiveness of these agents at normal doses, and those cells that do not possess these mechanisms may not respond. In high-dose therapy involving bone marrow transplants, the cell may be overwhelmed by the damage caused by these agent and die because of necrosis. The alkyl-ating agents are thought to be effective from G0-M and are, therefore, not considered cell cycle-specific agents. Many of the toxicities seen with the various agents are similar. Myelosuppression and gastrointestinal (GI) disruption, which often present as nausea and vomiting are commonly seen and are caused by the highly proliferative nature of these tissues, and therefore susceptibility to the effects of the alkylating agents. It is interesting to note that although cancer cells may develop resistance to the alkylating agents, this is not generally seen for cells of the bone marrow and GI tract because of the genetic stability and functioning DNA repair mechanisms present in these cells. However, an additional long-term consequence of the administration of these agents is the emergence of secondary cancers that are associated with the mutagenic effects of the alkylating agents themselves.

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