A new generation of oxazaphosphorine cytostatics with reduced toxicity, increased antitumor activity, antimetastatic efficacy and ability to cross the blood-brain barrier

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Oxazaphosphorine cytostatics (OX) such as cyclophophamide (CP) and ifosfamide (IF) are effective alkylating cytostatics that have been used clinically for over 50 years. However, all attempts to improve these "veterans" have so far been unsuccessful because her true mechanism of action was unknown. This in turn was because the results of in vitro tests were uncritically transferred to in vivo conditions. In doing so it was overlooked that in vitro, the pharmacologically active metabolite (OX-ALD) is converted into the alkylating metabolite OX-phosphoreamide mustard (OX-PAM) by β-elimination of acrolein. In vivo, however, no acrolein is formed because OX-ALD is cleaved enzymatically by phosphodiesterases into OX-PAM and the pro-apoptotic aldehyde 3-hydroxypropanal (HPA) [1] which - and this is special for OX - boosts the p53 dependent apoptosis initiated by OX-PAM.
Based on the results of toxicity tests according to which the OX metabolites OX-OH (4-hydroxy-CP, 4-hydroxy-IF) and chloroacetaldehyde are responsible for the toxicity of OX [2], thiazolidine- and perhydrothiazine derivatives were synthesized, which directly - bypassing the toxic metabolites - hydrolyze into the pharmacologically active metabolite OX-ALD. The thiazolidine and perhydrothiazine derivatives are 5-7 times less toxic than common OX such as CP and IF. A special property of the thiazolidine derivatives is that they cross the blood-brain barrier in a transport that can be inhibited by L-cysteine, so that these oxazaphosphorines can be used for the therapy of tumors of the central nervous system [3].
Since the cell death event in OX therapy, that is the p53-dependent apoptosis initiated by DNA damage, competes with the cell's own repair systems, apoptotic yield has to be increased by generation of irreparable DNA damage by OX with altered alkylating function. I-aldophosphamide perhydrothiazine, in which a chlorine atom of the alkylating function is substituted by a mesyl group, which creates irreparable DNA intrastrand crosslinks was constructed. This compound, called SUM-IAP, produces a 104-105-fold higher apoptosis in P388 tumors than the parent compound with unchanged alkylating function.
Experiments with SUM-IAP on advanced subcutaneously growing P388 tumors showed that after eradication of the primary tumor, the animals died 30-70 days after the start of therapy from the growth of SUM-IAP-resistant metastases. However, the animals survived without metastasis formation if they were treated with the apoptosis enhancer N-methylformamide immediately after SUM-IAP treatment [4].
It is well known that OX have an immune stimulating effect due to special sensitivity of T-cell inhibiting regulatory T-cells (Treg) to OX-OH. This immune stimulating effect can be used to prevent metastasis after SUM-IAP therapy by choosing the SUM-IAP regimen such that Tregs are killed by SUM-IAP-induced apoptosis as is shown in P388 tumor bearing mice [5].

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