The newer purine analogs. Significant therapeutic advance in the management of lymphoid malignancies

Systemic chemotherapy for the treatment of mycosis fungoides and Sezary syndrome

The treatment of mycosis fungoides and Sézary syndrome is unique. The treatments of choice are highly stage-and practitioner-dependent. For early stage patients, treatment with palliative topical therapies is often adequate to yield excellent, high-quality and durable responses. For the more advanced stages, systemic approaches are more appropriate, either alone or in combination, to palliate patients. There is still little convincing evidence from randomized trials that any single approach is favorable for improving survival. Many practitioners believe the disease can be controlled with the host immune system, but there again is little scientific support for this conclusion. It has been hypothesized that some of our therapies work through this mechanism, such as photopheresis and perhaps even psoralen and ultraviolet A. Unfortunately, even the use of biologic response modifiers, such as interferon and the interleukins, is not absolutely supportive of an active role for immune surveillance since these agents have been shown to have cytotoxic and differentiating effects. Because of the above, many practitioners have actively discouraged the use of chemotherapy because it may impair the host immune system; certainly, the purine analogs would fall into this category. However, there is little evidence supporting significant immune effects for other chemotherapeutics, and clearly, the responses seen would support their use in appropriate patients. As the present authors will detail in this paper, the advances in understanding cancer biology and mechanisms of resistance should, in the future, lead to optimal selection of agents that are predicted to be optimally active, limiting the toxicity and waste associated with ineffective drug usage.

Fludarabine and cytarabine as a sequential infusion regimen for treatment of adults with recurrent, refractory or poor prognosis acute leukemia

We did a retrospective analysis on the safety and efficacy of sequential infusion fludarabine and cytosine arabinoside (ara-C) in treating refractory, recurrent or poor prognosis acute leukemia in adult patients. Forty-five adult patients with acute myelogenous leukemia (AML) or acute lymphoblastic leukemia (ALL) received a total of 68 courses of sequential continuous infusion of fludarabine for 2 days (total dose 71.5 mg/m(2) ) followed by 3 days of ara-C (total dose 7590 mg/m(2) ). Thirty-nine patients had refractory or recurrent disease, and six had other adverse prognostic features. Thirty-six patients had AML, seven had ALL, and two had CML in blastic phase. Complete remission was seen in 20 patients (44%), and partial remission in 5 patients (11%), giving a total response rate of 56%, similar for both AML and ALL. Duration of response to prior therapy did not affect the response rate. All 3 patients with Philadelphia chromosome positive ALL obtained complete remission. Median remission duration was 4.7 months (range 0.6-36.6), and median overall survival was 5.0 months (0.7-40+). Median overall survival was 10.1 months in responders. Pulmonary toxicity was seen in 8 patients, of whom 2 died from adult respiratory distress syndrome. No cardiac toxicity was observed, but 3 patients had transient cerebellar toxicity. Profound myelosuppression was seen in all patients. We conclude that the sequential infusion of fludarabine and ara-C is an effective non-cardiotoxic regimen for adults with refractory, recurrent or poor prognosis acute leukemia, may be particularly useful for resistant Philadelphia chromosome positive ALL, and may warrant further investigation in this subset. Pulmonary rather than neurological toxicity may be a unique side effect of the regimen.

Involvement of multiple transporters in the oral absorption of nucleoside analogues

Many nucleoside analogues such as azt, ddI, ddC, d4T, 3TC, acv and vacv are currently being used in the treatment of patients infected with HIV, suffering from AIDS, or AIDS-related opportunistic infections. The transport of nucleoside analogues across the gastrointestinal tract is mediated by a number of transporters that fall into three broad categories, i.e., Na(+)-dependent concentrative transporters, Na(+)-independent equilibrative transporters and H(+)/peptide transporters. The first two transporter classes contain a large number of subtypes that are based on the substrate specificity. Recent studies have shown that most of the anti-HIV nucleoside analogues are transported by one or more of the nucleoside transporters. Furthermore, certain analogues, such as acv, appear to be absorbed by non-carrier-mediated diffusion, whereas vacv is apparently transported by non-nucleoside transporters (e.g., the oligopeptide transporter, PepT1 and possibly others). Thus, it is desirable to understand the precise nature of the absorption mechanism of these drugs to improve bioavailability and reduce the variability that is commonly observed in vivo in human patients. A complete understanding of the complex interactions of nucleoside analogues with the various transporters will help in designing better delivery systems and strategies to improve efficacy. In the current report, the mechanisms of nucleoside and nucleoside-analogue transport are reviewed. Also, methods of exploiting prodrugs to improve the bioavailability characteristics of drugs are highlighted.

Comparative effects of cladribine, fludarabine and pentostatin on nucleotide metabolism in T- and B-cell lines

BACKGROUND AND AIMS

the purine nucleoside analogues cladribine (CdA), fludarabine (F-Ara-AMP) and pentostatin (dCf), are effective therapy for a range of T- and B-cell lymphoid malignancies. The effects upon nucleotide metabolism in human CCRF-CEM T-cell leukaemia and Raji B-cell lymphoma cell lines of these drugs have been compared to assess possible mechanisms of cytotoxicity.

METHODS

Leukaemia cells were exposed to a purine nucleoside analogue and perchloric acid extracts were analysed by HPLC for 2'-deoxynucleoside-5'-triphosphates (dNTPs), nucleoside-5'-triphosphates (NTPs) and drug metabolites.

RESULTS

After addition of a purine nucleoside analogue, CdA-TP and F-Ara-ATP accumulate in cells while the levels of dCf-TP formed were not detectable by ultra-violet absorbance. In response to accumulating concentrations of drug triphosphate, the cellular levels of dNTPs initially decrease (0-4 h), then accumulate above their initial levels (4-10 h) before slowly declining beyond 10 h. NTPs also accumulate during the period 4-10 h before declining at later times.

CONCLUSION

The temporal effects on the levels of dNTPs and NTPs of the 3 purine nucleoside analogues are similar against CCRF-CEM and Raji cells. However, CdA induces major depletions of dTTP, dGTP and dATP in CCRF-CEM cells and F-Ara-A induces a major accumulation of dATP in Raji cells.

Functional and molecular characteristics of Na(+)-dependent nucleoside transporters

Nucleoside transporters play a critical role in the absorption, disposition, and targeting of therapeutically used nucleosides and nucleoside analogs. This review is focused on the Na(+)-dependent, concentrative nucleoside transporters which are found in a variety of cells including renal, intestinal and hepatic epithelia. Five major Na(+)-dependent nucleoside transporter subtypes have been characterized in isolated tissue preparations: N1 is purine selective; N2 is pyrimidine selective and N3-N5 exhibit variable selectively for both purine and pyrimidine nucleosides. The recent cloning of N1 and N2 nucleoside transporters has provided the first information on the molecular function and structure of concentrative nucleoside transporters. In this manuscript we review the characteristics of the various subtypes of nucleoside transporters and the molecular structure, functional properties, and tissue distribution of the cloned Na(+)-dependent nucleoside transporters. In addition, the interactions of nucleosides and nucleoside analogs with the cloned transporters in mammalian and amphibian expression systems are presented. Mammalian expression systems may be particularly useful during drug development in screening potential compounds for improved bioavailability and tissue specific targeting. Finally, we present our view of future ares of study in the field of nucleoside transporters.

Pentostatin (2'-deoxycoformycin) in the treatment of cutaneous T-cell lymphoma

BACKGROUND

The treatment of patients with advanced or therapy-refractory cutaneous T-cell lymphoma (CTCL) remains a challenge. Pentostatin is a potent inhibitor of adenosine deaminase and is selectively toxic to lymphocytes. In a small number of patients with CTCL, it previously has been shown to be effective.

OBJECTIVE

Our purpose was to evaluate the efficacy and safety of pentostatin in the treatment of patients with advanced and/or therapy-refractory CTCL.

METHODS

Eighteen patients with stage I to IVb CTCL were treated with 4 to 5 mg/m2 of intravenous pentostatin every 1 to 4 weeks.

RESULTS

Two patients (11%) had complete responses of 4 months and 6 years, respectively. These patients had stage III and IVa CTCL and had previously received many different external or systemic treatments. Partial remission (50% to 99% clearing) lasting for 1.5 to 6 months occurred in five patients (28%) with stage IIa (n = 3), stage IIb, and stage IVa CTCL. These patients had received a median of three prior external or systemic treatments. No major side effects were observed, and bone marrow suppression was mild.

CONCLUSION

Single-agent pentostatin in intravenous doses of 4 to 5 mg/m2 is an effective systemic treatment of CTCL (39% objective response rate) with little toxicity.

Chemotherapeutic purine analogs alter the level of interferon-beta mRNA induced by poly I-poly C in cultured osteosarcoma cells

The purine nucleoside analogs fludarabine, 2-chlorodeoxyadenosine, and 2'-deoxycoformycin exhibit impressive activity in lymphoproliferative malignancies of adults and children. Their mechanism of action is not clear. Studies have suggested that their use is associated with significant myelosuppression, immunosuppression, and in some circumstances, increased infection with viral and opportunistic pathogens. Because interferons (IFNs) are known to have immunomodulatory activity as well as potent antiproliferative and antiviral activity, we examined whether the chemotherapeutic purine nucleoside analogs alter interferon-beta (IFN-B) gene expression in MG63 in human osteosarcoma cells. Northern blot analysis showed a dose-dependent inhibition of IFN-B mRNA accumulation in response to a known inducer (Poly I-Poly C) all three purine analogs. Hybridization analysis also revealed that inhibition of IFN-beta mRNA accumulation by the purine analogs is not a result of decreased mRNA stability. Further analysis of gene expression by PCR differential display indicated that the effect of the purine analogs was restricted to only a limited number of inducible genes. The data suggest that these molecules alter the signaling process involved in regulating the expression of specific genes, including IFN-beta. These findings predict that the use of purine nucleoside analogs may reduce IFN production in vivo and thereby abrogate host defenses against infectious pathogens.

2-Chlorodeoxyadenosine in the treatment of relapsed/refractory chronic lymphoproliferative disorders

2-Chlorodeoxyadenosine (2-CdA) is a purine analog with cytotoxic activity on both resting and cycling lymphocytes which has been used as salvage therapy in advanced/resistant chronic lymphoproliferative disorders. In our study 39 patients (19 B-CLL, 5 B-PLL, 9 low-grade B-NHL, 5 CTCL and 1 high-grade T-NHL) who relapsed or became resistant after 1-4 chemotherapy regimens were treated with 2-CdA 6 mg/m2 per day by 2 h infusion for 5 d every 28 d. The overall clinical response rate, including complete remission (CR) and partial remission (PR), was 66%. Two of 19 (10%) B-CLL patients achieved a CR lasting 9 months, while 11/19 B-CLL (58%) and 4/5 B-PLL (3 B-PLL/B-CLL and 1 B-PLL) (80%) achieved a PR. Interestingly, 5 of 6 patients who had been previously treated with fludarabine obtained a clinical response (2 CR and 3 PR). One of 9 (11%) low-grade B-NHL patients achieved a CR and relapsed after 26 months, and 5/9 (55%) achieved a PR. One of 5 (20%) CTCL achieved a CR lasting 32 months, while 2/5 (40%) achieved a PR. The overall mean duration of PR was 7.4 months and no differences were observed among different groups of patients. Toxicity was acceptable, as only a transient severe hematological impairment was observed in 20% of the patients while nonhematological toxicity was not documented. Two patients died because of bacterial pneumonia, 1 of meningitis due to Listeria and 9 from progression of the disease. In conclusion, treatment with 2-CdA in heavily pretreated patients with chronic lymphoproliferative disorders is well tolerated and obtains high response rates, even in patients relapsed after treatment with fludarabine.

Purine nucleoside therapy of low-grade follicular lymphoma

Low-grade follicular lymphomas, follicular small cleaved and follicular mixed, generally follow an indolent but progressive course. Although available therapies induce responses, continuous relapse occurs. In investigating potential therapeutic regimens, researchers have sought regimens that will result in a postchemotherapy survival plateau rather than a pattern of continuous relapse. The newer, purine analogs, 2'-deoxycoformycin, fludarabine, and 2-chlorodeoxyadenosine have shown activity in the low-grade B-cell lymphomas, especially with the follicular histologic subtype. Response rates in studies of previously untreated patients approach 100%. The newer purine analogs are emerging as an important treatment approach for indolent B-cell lymphomas, but the impact on survival remains to be assessed.

Non-Hodgkin's lymphomas--current status of therapy and future perspectives

Non-Hodgkin's lymphomas (NHL) are a heterogeneous group of disorders which can either be classified according to their biology, represented by corresponding counterparts of normal lymphocyte development as in the Kiel classification, or according to their clinical course, used in the Working Formulation. The recently proposed Revised European-American Lymphoma (R.E.A.L.) classification may unify both aspects and facilitate the comparability of international studies. Besides histology, the extent of disease still comprises the major determinant of therapy. In high-grade lymphomas combination chemotherapy with cyclophosphamide, hydroxydaunorubin, vincristine and prednisone (CHOP) represents the treatment of first choice, and may be restricted to 3-4 cycles in patients with limited stages of the disease when followed by involved field radiotherapy. In more extended, bulky stage II to IV disease, treatment must be extended to six courses of CHOP and, potentially, additional irradiation. Even in advanced states of the disease, long-term remission and potential cure are achieved in 30-50% of cases. In low-grade lymphomas, most patients present with advanced stages III and IV for which chemotherapy can be applied with palliative intention only. Hence, a watch-and-wait approach still seems appropriate outside clinical investigations until the disease requires a therapeutic intervention. This consists preferentially of chemotherapy of moderate intensity such as cyclophosphamide, vincristine and prednisone (COP) or prednimustine and mitoxantrone (PmM). In responding patients, maintenance therapy with interferon-alpha is currently being explored and may result in prolongation of disease-free and, possibly also, overall survival. In both high- and low-grade lymphomas, intensification of therapy by myeloablative chemotherapy or combined chemoradiotherapy followed by autologous bone marrow transplantation (ABMT) or peripheral stem cell transplantation provides a promising and potentially curative prospective. In addition, new cytostatic agents such as the purine analogues--fludarabine, chlorodeoxyadenosine and deoxycoformycin--enlarge the therapeutic spectrum. More experimental approaches consist of the application of immunotoxins or radioisotypes, coupled to monoclonal antibodies directed against lymphoma-specific antigens. Overall, the substantial advances that have been achieved in the understanding of the biology and pathogenesis of malignant lymphomas, as well as the current achievements of therapy and the new promising perspectives, justify the hope that curative therapy can soon be offered to an increasing proportion of patients with NHL.

The influence of 2-chloro-2'-deoxyadenosine on metabolism of deoxyadenosine in human primary CNS lymphoma

The effects of 2-chloro-2'-deoxyadenosine (2CdA) on the activity of enzymes important for the metabolism of deoxyadenosine were studied in lysates prepared from human primary central nervous system (CNS) lymphomas and normal human lymphocytes. Strong inhibition (approximately 100%) of the phosphorylation of deoxyadenosine to its deoxynucleotide phosphate derivatives was produced in both systems in the presence of 2CdA, which was phosphorylated concomitantly to 2-chloro-2'-deoxyAMP. Interestingly, 2CdA was also found to be an inhibitor of the deamination of both deoxyadenosine (over 50%) and AMP (70%). These findings add to our understanding of the mechanisms of toxicity of this drug, especially considering that 2CdA is resistant to deamination by adenosine deaminase. These results challenge the existing theories of 2CdA toxicity, which have been limited to the formation of phosphate derivatives of 2CdA. The present in vitro studies have demonstrated that 2CdA also inhibits both phosphorylation and deamination of deoxyadenosine (dAdo), suggesting that its mechanism of toxicity includes a block in dAdo metabolic pathways. This has important implications for the perturbation of cell methylation, a functionality associated with, for example, apoptosis.

Bone marrow transplantation in low-grade non-Hodgkin's lymphoma

The low-grade histologic types constitute one quarter of all non-Hodgkin's lymphomas (NHL). Conventional chemotherapy and chemo-radiation therapy have failed to significantly alter the course of this disease, and most patients eventually succumb to lymphoma. Despite the fact that NHLs exhibit a steep dose-response relation to cytotoxic therapy, fewer than 30% of eligible patients undergo bone marrow transplantation. Reasons for fewer patients receiving this course of treatment include: elderly patient population, extensive previous chemotherapy and/or radiation therapy, high incidence of bone marrow involvement, and transformation to higher grade NHLs. In recent years, improvements in several areas have enhanced the therapeutic index for bone marrow transplantation. These advances include the use of more effective preparative regimens, recombinant hematopoietic growth factors, extended-spectrum antibiotics, and an increased expertise in blood transfusion techniques and practices. Other, more effective strategies include sophisticated in vitro bone marrow purging approaches and peripheral blood progenitor cell collection. As a result, more patients have been able to receive dose-intensive therapy followed by hematopoietic cellular rescue. Although follow up is short in most series, encouraging results have stimulated some centers to begin transplanting responding patients earlier in their disease course; in more than 200 patients treated in this fashion, long-term disease-free survival has been achieved in nearly 70% of patients, some patients for a period of greater than 6 years. The new purine analogues fludarabine, pentostatin, and 2-chlorodeoxyadenosine also have shown promise in both initial and salvage treatment of low-grade NHLs. It remains to be determined whether this group of drugs will be complimentary to the bone marrow and/or peripheral blood progenitor cell transplant approach.