Mechanisms of resistance to low-dose metronomic cyclophosphamide

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Background: It has been postulated that antiangiogenic therapies are less susceptible to acquired resistance compared to conventional treatment strategies, which target genetically unstable tumor cells. However, resistance is commonly seen (pre)clinically. A better understanding of the underlying mechanisms is clearly needed to improve such therapies. Methods: We studied mechanisms of acquired resistance to low-dose metronomic (LDM) cyclophosphamide (CTX), the continuous or frequent administration of comparatively low doses of CTX with no extended breaks resulting primarily in antiangiogenic effects, by using LDM CTX relapsed/resistant variants obtained during prolonged in vivo therapy from the human prostate cancer cell line PC-3, which initially is highly responsive. The variants were analyzed for their growth characteristics, angiogenic “phenotype” and impact on circulating endothelial cells (CEC)/endothelial cell progenitors (CEP). Furthermore, we obtained microvessel density (MVD) counts of corresponding xenografts and analyzed in vitro resistance to 4-hydroxycyclophosphamide (4-OH-CTX), hypoxia, acidosis, and glucose as well as serum deprivation. Results: The resistant variants did not show consistent changes in their angiogenic “phenotype”. The decrease in MVD upon LDM CTX treatment was comparable to that obtained in xenografts from parental PC-3, which was reflected by similar changes in the number of CEC and CEP. Despite in vivo resistance to LDM CTX, there was no evidence of in vitro resistance of the variants to 4-OH-CTX. However, they exhibited increased tolerance to microenvironmental stress resulting from sustained antiangiogenic activity. Conclusions: In this model, resistance to LDM CTX develops despite sustained antiangiogenic activity. This provides further evidence that reduced vascular dependence of the tumor cell population may be a basis for the observed resistance. The administration of CTX in a LDM manner does not result in tumor cell CTX resistance, supporting the notion that LDM protocols target primarily the tumor vasculature. If confirmed in ongoing clinical metronomic chemotherapy trials, our results will have major implications for the management of patients that progress under such treatment.

No significant financial relationships to disclose.

Adrenal insufficiency with enlarged adrenals

Five patients with adrenal insufficiency and large adrenal glands at presentation are reported. Addison's disease was due to adrenal tuberculosis in three patients, with important changes in adrenal configuration on CT reflecting the natural history of the disease. Adrenal infiltration by non-Hodgkin lymphoma and metastatic carcinoma of the lung was the cause of the disease in the fourth and fifth patients, respectively, who developed signs of adrenal insufficiency before the diagnosis of the primary lesion became apparent. Histologic confirmation was established after unilateral adrenalectomy in three patients. In two patients with adrenal tuberculosis, long clinical and laboratory follow-up confirmed the diagnosis. This report indicates that Addison's disease is not infrequently associated with adrenal enlargement. Adrenal size is related to the cause and duration of the various disease states leading to adrenal insufficiency. Moreover, adrenal insufficiency associated with enlarged adrenal glands can be the presenting manifestation of lymphoma or metastasis.