Effect of calcium channel blockers on human CFU-GM with cytotoxic drugs

Beyond biologics: is there a role for systemic cytotoxic chemotherapy in neuroendocrine tumors?

Abstract: 

Neuroendocrine tumors (NETs) are a group of biologically and clinically heterogeneous neoplasms that have varying degrees of responsiveness to systemic and targeted therapies. With recent advances in therapeutics, a number of biologic therapies have been incorporated into the management of NETs. In this review, we discuss the role of cytotoxic chemotherapy in the management of NETs with an overview of past and present clinical trials, current treatment guidelines and future directions.

Inhibitory effects of verapamil isomers on the proliferation of choroidal endothelial cells

PURPOSE

To evaluate the effects of verapamil isomers on in vitro proliferation of bovine choroidal endothelial cells (CECs).

MATERIALS AND METHODS

CECs were isolated from bovine eyes and cultured in endothelial growth medium (EGM). For the proliferation assays, CECs were exposed to verapamil isomers (0.1-100 microM) in EGM with 2% fetal bovine serum or basic fibroblast growth factor (bFGF) (10 ng/ml). After 72 h of incubation with the desired drug, the cellular proliferation was determined by an MTT assay and a BrdU assay. In addition, the drug toxicity on CECs stimulated with EGM was evaluated by cell counting with trypan blue.

RESULTS

All verapamil isomers inhibited the bFGF- or medium-stimulated growth significantly in a concentration range of 10-40 microM without toxicity. No significant differences were seen between the inhibitory effects of the various isomers. Cell toxicity was detected at a concentration of 100 microM verapamil isomers on EGM-stimulated CECs.

CONCLUSION

The results demonstrate the efficacy of all verapamil isomers in inhibiting CEC proliferation involved in the process of choroidal neovascularization. D: -(+)-Verapamil may be recommended for further in vivo evaluation in an animal model of exudative AMD; it has fewer systemic and local side effects because calcium channels are not blocked.

Comparison of a neural net-based QSAR algorithm (PCANN) with Hologram- and multiple linear regression-based QSAR approaches: application to 1,4-dihydropyridine-based calcium channel antagonists

A QSAR algorithm (PCANN) has been developed and applied to a set of calcium channel blockers which are of special interest because of their role in cardiac disease and also because many of them interact with P-glycoprotein, a membrane protein associated with multidrug resistance to anticancer agents. A database of 46 1,4-dihydropyridines with known Ca2+ channel binding affinities was employed for the present analysis. The QSAR algorithm can be summarized as follows: (1) a set of 90 graph theoretic and information theoretic descriptors representing various structural and topological characteristics was calculated for each of the 1,4-dihydropyridines and (2) principal component analysis (PCA) was used to compress these 90 into the eight best orthogonal composite descriptors for the database. These eight sufficed to explain 96% of the variance in the original descriptor set. (3) Two important empirical descriptors, the Leo-Hansch lipophilic constant and the Hammet electronic parameter, were added to the list of eight. (4) The 10 resulting descriptors were used as inputs to a back-propagation neural network whose output was the predicted binding affinity. (5) The predictive ability of the network was assessed by cross-validation. A comparison of the present approach with two other QSAR approaches (multiple linear regression using the same variables and a Hologram QSAR model) is made and shows that the PCANN approach can yield better predictions, once the right network configuration is identified. The present approach (PCANN) may prove useful for rapid assessment of the potential for biological activity when dealing with large chemical libraries.

Calcium channel antagonists inhibit growth of subcutaneous xenograft meningiomas in nude mice

BACKGROUND

We have previously shown that calcium channel antagonists inhibit in vitro meningioma growth. This study examines the effect of calcium channel antagonists on in vivo xenograft meningioma growth.

METHODS

Meningioma cells taken from human patients were mixed with Matrigel and injected into the subcutaneous space in the flank of nude mice. These animals were treated with calcium channel antagonists in their drinking water. Tumor volumes were measured over time; comparison was made between control and treatment groups. Daily weights, average daily water consumption, and serum calcium channel antagonist levels were determined. Comparison of histology and proliferation index was made between control and treatment groups.

RESULTS

Diltiazem treatment decreased tumor growth over time compared to control groups. Increased tumor growth inhibition was seen with increasing doses (p > 0.05). Treatment with verapamil had similar effects; however, there are no statistically significant dose dependent decreases in growth with increasing verapamil doses. There were no tumor "cures" or spontaneous regression of tumor in any group including the control groups. Animal daily weight and average daily water consumption was unaffected by increasing calcium channel antagonist doses compared to control groups. Mouse serum drug levels increased with increasing doses of drug in the drinking water of treatment groups (p > 0.05). Histology and proliferative index of treatment groups were similar to control groups.

CONCLUSION

Calcium channel antagonists decrease but do not completely inhibit the growth of meningiomas in nude mice. Clinical correlations and potential applications are discussed.

Verapamil inhibits proliferation, migration and protein kinase C activity in human retinal pigment epithelial cells

The effects of three calcium channel blockers, verapamil, diltiazem and nifedipine, were examined on in vitro proliferation and migration of human retinal pigment epithelial cells. Human retinal pigment epithelial cells were seeded in Dulbecco's modified essential medium with 10% fetal bovine serum and different concentrations of the three calcium channel blockers. After 3 days of treatment, cell proliferation was determined by cell counting and by [3H]-thymidine uptake. Cell viability was determined with trypan blue exclusion. For determination of cell migration, retinal pigment epithelial cells were grown to confluence and then growth-inhibited with mitomycin C. After a 3 mm zone was denuded, the cells were treated with different concentrations of the calcium channel antagonists. After 24 hr, the cells that had migrated over the wound edge were counted. To determine the involvement of protein kinase C in the verapamil effect, its activity was measured in both verapamil-treated and untreated cells. Verapamil dose dependently inhibited serum-induced proliferation of retinal pigment epithelial cells, when measured by cell number (IC50 14.6 microM) or [3H]-thymidine incorporation (IC50 11.3 microM). At concentrations of 15 microM and below, there was no effect on cell viability, as determined by morphology and trypan blue exclusion. Diltiazem inhibited cell proliferation at a concentration of 100 microM; however, 100 microM nifedipine had no effect. Verapamil showed a significant inhibition of serum-induced migration in the range of 10 microM to 0.1 microM. The IC50 of the inhibition of retinal pigment epithelial cell proliferation and migration by verapamil is significantly higher than that seen for effects on calcium channel blockage. Eight micromolar verapamil reversibly inhibited total protein kinase-C activity in retinal pigment epithelial cells suggesting the possibility that the drug may act by inhibiting the protein kinase-C pathway. These data suggest the potential of the calcium channel blocker verapamil as a pharmacological modulator of disorders such as proliferative vitreoretinopathy in which there is increased retinal pigment epithelial cell proliferation and migration.

P-glycoprotein, multidrug resistance and protein kinase C

The multidrug resistant (MDR) phenotype is a well-studied subject that has been recognized as a determinant underlying specific types of drug resistance in human cancer. Although it is clear that the P-glycoprotein plays a major role in MDR, it is not clear whether post-translational modifications such as phosphorylation have any major impact on its modulation. The laboratory of Dr. Bruce Chabner was one of the first to describe increased expression and activity of protein kinase C (PKC) associated with the MDR phenotype. Since that time, a similar correlation has been observed in many other MDR cell lines. Most of these studies have been performed with doxorubicin-selected cells that have acquired MDR and have shown increased PKC activity, mainly for PKC-alpha isoenzyme. Intrinsic MDR in human renal cell carcinoma lines has been shown to correlate directly with PKC activity, but further studies with intrinsic MDR cell lines are needed before any conclusions can be drawn. More recent evidence suggests that there is a complex biochemical process by which PKC isoenzymes differentially phosphorylate specific serine residues in the linker region of P-glycoprotein which may lead to alterations in P-glycoprotein ATPase and drug-binding functions. To further complicate matters, PKC plays an important role in anti-apoptotic pathways, which can confound the dissection and elucidation of drug-resistance mechanisms. However, these areas are still under active investigation and not fully answered. Further studies are needed to specifically answer the question of whether PKC directly modulates basal and/or drug-stimulated P-glycoprotein function. This manuscript reviews the majority of the literature on PKC and MDR, as well as offers caveats for interpretation of these studies to answer the above questions.

Inhibition of in vitro meningioma proliferation after growth factor stimulation by calcium channel antagonists: Part II--Additional growth factors, growth factor receptor immunohistochemistry, and intracellular calcium measurements

We have previously reported that calcium channel antagonists can block both the growth of meningiomas in culture and the potent growth stimulation of meningioma cells by epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). This study further defines the nature of this growth inhibition. Primary meningioma cultures were established, and cells were characterized. Fibroblast growth factor or insulin-like growth factor-I growth stimulation in the presence of calcium channel antagonists was examined. In addition, the effects of ethylene glycol-bis-(aminoethylether) N,N,N',N"-tetraacetic acid and Bay K 8644, a calcium channel agonist, on the growth factors were analyzed. Growth factor receptor immunohistochemistry was performed on the original tumors and the in vitro meningioma cells. Twelve of 17 (71%) meningiomas in this study were positive for the EGF receptor, and 14 of 17 (82%) were positive for the PDGF receptor. Five of six (83%) of the culture cells were positive for the EGF receptor, and four of five (80%) were positive for the PDGF receptor. Intracellular calcium changes were quantified using the intracellular calcium-chelating, fluorescent dye, Fura-2. The growth stimulation of fibroblast growth factor and insulin-like growth factor-I on meningioma cells in culture was decreased in a dose-dependent manner by calcium channel antagonists. The growth stimulation of fibroblast growth factor and insulin-like growth factor-I was not affected by a reduction of extracellular calcium, whereas the growth stimulation of EGF and PDGF was. Interestingly, intracellular calcium was not increased after exposure to growth factors but was increased after serum stimulation. This increase could be blocked by preincubation with verapamil. Calcium channel antagonists can inhibit proliferation of meningioma cells in culture after stimulation with a number of growth factors. These drugs might disrupt intracellular calcium homeostasis or interfere with key elements of the growth factor signal transduction pathways. These mechanisms as well as the potential clinical relevance of these findings are discussed.

Effect of cyclosporin A on human bone marrow granulocyte-macrophage progenitors with anti-cancer agents

Cyclosporin A (CyA) overcomes P-glycoprotein (P-gp) associated multidrug resistance (MDR). P-gp expression is frequently observed among, not only various cancer cells, but also several normal tissues including bone marrow progenitor cells. These findings lead us to examine whether CyA enhances the myelotoxicity of anti-cancer agents. Bone marrow mononuclear cells were incubated with anti-cancer agents (vincristine, VCR; doxorubicin, ADM; etoposide, VP-16; cytarabine, Ara-C; methotrexate, MTX) and a concentration of CyA (0.5, 5.0 micrograms/mL). The methylcellulose assay for granulocyte-macrophage progenitors (CFU-GM) was conducted using the post-treated cells. There was no significant toxicity for marrow CFU-GM formation after 72 h incubation with CyA (84-108% of control). The inhibitory concentration that reduced colonies by 50% (IC50) was 12 nmol/L for VCR, 6 nmol/L for ADM, 220 nmol/L for VP-16, 15 nmol/L for Ara-C and 35 nmol/L for MTX, respectively. For VCR, ADM and VP-16, the number of CFU-GM was unchanged with the addition of CyA at 0.5 microgram/mL concentration. In contrast at 5 micrograms/mL CyA, the number of CFU-GM (% of control) was reduced significantly (P < 0.05 or P < 0.01). With MTX and Ara-C, the number of CFU-GM was unchanged after addition of CyA, even at 5 micrograms/mL concentration. We conclude CyA may therefore enhance cytotoxic drug sensitivity in MDR tumor cells at a clinically achievable concentration (0.5 microgram/mL) without marrow toxicity.

In vitro growth inhibition of growth factor-stimulated meningioma cells by calcium channel antagonists

Studies have shown that a majority of meningiomas contain receptors for platelet-derived growth factor and epidermal growth factor and that these growth factors promote the proliferation of meningioma cells in culture. Although the mechanism of action has not been elucidated, intracellular calcium appears to be part of the signal transduction mechanism. Because alterations in intracellular calcium could interrupt this pathway and decrease cellular proliferation, we investigated the effects of calcium channel-blocking agents on the growth of meningioma cells in vitro. Primary meningioma cell cultures were established, and the cells were characterized by light and electron microscopy and by immunohistochemical studies. Then, the cultures were given growth factors and/or various calcium channel antagonists, and growth rates were measured. A dose-response decrease in cell growth was seen when verapamil, nifedipine, or diltiazem (voltage-dependent calcium channel-blocking agents) was added to serum-containing media. Also, these drugs blocked the growth stimulation of epidermal growth factor and platelet-derived growth factor in a similar fashion. Dantrolene, which inhibits the release of sequestered intracellular calcium, was also an effective blocker of the mitogenic stimulation of these growth factors.

Variable effects of tamoxifen on human hematopoietic progenitor cell growth and sensitivity to doxorubicin

To determine the influence of tamoxifen on the drug sensitivity of normal human hematopoietic progenitor cells, T-cell- and adherent-cell depleted human bone marrow mononuclear cells (T-, Ad-) were exposed in vitro to 5 microM tamoxifen for 24 h. The effects of tamoxifen were highly variable, as exposure to tamoxifen produced an increase (97% +/- 12.3%) in the growth of day-12 committed myeloid progenitors (CFU-GM) in only four of ten experiments utilizing bone marrow from different donors. When T-, Ad- myeloid progenitor cells treated with tamoxifen were subsequently exposed to doxorubicin, 7 of 14 experimental samples studied demonstrated a net increase in the number of surviving clonogenic cells as compared with cells exposed to doxorubicin alone. Tamoxifen also stimulated the growth of a more purified (CD34(+)-selected) progenitor cell population in four of four experiments (by 62.5% +/- 4.9%) but did not increase the survival of these cells upon exposure to doxorubicin; in fact, in five of ten experimental samples, tamoxifen enhanced cell sensitivity to doxorubicin. Taken together, these observations indicate that tamoxifen produces variable stimulation of committed myeloid progenitor cell growth in vitro. Furthermore, while under some circumstances, tamoxifen appears to have the capacity to enhance CFU-GM survival in the presence of doxorubicin, this drug combination may also result in enhanced toxicity to normal bone marrow progenitors.

A randomised clinical study of verapamil in addition to combination chemotherapy in small cell lung cancer. West of Scotland Lung Cancer Research Group, and the Aberdeen Oncology Group

Proliferation of drug resistant tumour following chemotherapy is the principal cause of treatment failure in small cell lung cancer (SCLC). Verapamil has been shown to partially restore drug sensitivity in tumour cells rendered resistant in vitro. The results of the first large-scale randomised study of a resistance modifying drug given in conjunction with chemotherapy in cancer patients are reported. Two hundred and twenty-six patients have been entered. All patients received four cycles of cyclophosphamide (750 mg m-2), doxorubicin (40 mg m-2) and vincristine (1.4 mg m-2) on Day 1 and etoposide (75 mg m-2) on Days 1, 2 and 3, repeated at 21 day intervals. Those patients randomised to the verapamil arm received oral verapamil 120 mg qid for 5 days with each course of chemotherapy. Similar numbers of cycles of protocol treatment were given in both arms with over 75% of patients completing all four cycles. There were no significant differences in general toxicities between the two arms, except for more severe alopecia in the verapamil treatment group (P = 0.045). There was no significant difference in cardiovascular or haematological toxicity, although the median nadir white cell count after Cycle 1 chemotherapy was lower in the verapamil arm (P = 0.065) and there were significantly more dose reductions after Cycle 1 in the verapamil arm (P = 0.031). No statistically significant differences in response (P = 0.582) or survival (P = 0.290) data were seen. The absence of a significant improvement in response or survival using verapamil may relate to the low blood levels of verapamil seen in the clinic (0.8 microM), in contrast to those known to be maximally active in vitro (> 6 microM) or to the presence of other cellular mechanisms by which drug resistance develops.

Multidrug resistance in acute leukemia: a conserved physiologic function

Native resistance to conventional chemotherapy remains an important cause of treatment failure in the adult acute leukemias. Delineation of cellular mechanisms of drug resistance therefore represents a prerequisite to the development of more effective treatment strategies. The multidrug resistance (MDR) phenotype represents one such mechanism of resistance with direct clinical relevance. This phenotype occurs normally in certain mammalian tissues, and is detectable in tumor cell lines selected for resistance to naturally occurring antineoplastics. The mdr1 gene or its glycoprotein product, P-glycoprotein, is detected with high frequency in secondary acute myeloid leukemia (AML) and poor-risk subsets of acute lymphoblastic leukemia. In prospective studies in AML, MDR overexpression is an independent determinant of response to treatment and overall survival with conventional-dose induction regimens. Investigations of mdr1 regulation in normal hematopoietic elements has shown a pattern which corresponds to its regulation in acute leukemia, explaining the linkage of mdr1 to specific cellular phenotypes. Therapeutic trials are now in progress to test the ability of various MDR-reversal agents to restore chemotherapy sensitivity in high-risk acute leukemias.

In vitro and in vivo modulation of multi-drug resistance with amiodarone

The modulating effect on drug resistance of amiodarone (AM) and its metabolite desethylamiodarone (DEA) was studied in a P-glycoprotein-positive human colon carcinoma cell line COLO 320, and a human small-cell lung carcinoma cell line GLC4 and its adriamycin (Adr)-resistant subline GLC4-Adr (both P-glycoprotein-negative). AM, DEA and verapamil induced an increase in cytotoxicity of Adr, vincristine and etoposide (VP16) in COLO 320 cells, while in the GLC4 and GLC4-Adr cell line no effect was seen. In the COLO 320 cell line, AM caused more intracellular, and especially intranuclear, fluorescence of Adr and more Adr-induced DNA strand breaks as compared to Adr alone. Moreover, an increase in VP16-induced topoisomerase II-DNA complexes was observed when AM was added. Competition between AM and Adr for the same efflux pump was suggested in efflux studies. The colony-forming unit granulocyte macrophage (CFU-GM) assay showed no increase in cytotoxicity of Adr when AM was added. Fourteen patients with Adr-resistant tumors were treated with Adr and AM. In these patients, peak serum levels of AM plus DEA of 10 microM were reached. Patient serum (20%) obtained after the first i.v. AM infusion induced in vitro significantly more cell kill of Adr in COLO 320 cells. Apart from a transient first-degree AV block in one patient, no cardiac toxicity was observed with the combination of Adr and AM. Bone-marrow toxicity was the same as expected from Adr alone in these patients. One of the 13 evaluable patients obtained a partial remission.

Differential in vitro sensitivity of human tumor and normal cells to chemotherapeutic agents and resistance modulators

The intrinsically Vincristine(Vcr)-resistant human kidney adenocarcinoma cell line ACHN, the human acute lymphoblastic leukemia cell line L0, its more-than-100-fold Vcr-resistant subline LI00, normal human fibroblasts and lymphocytes, also tumor cells from patients with chronic lymphocytic leukemia (CLL), acute myeloblastic leukemia (AML) and solid tumors, were compared for sensitivity to cytotoxic drugs and resistance modulators (RMs). The LI00 cells showed pronounced sensitivity to the RMs verapamil (Ver), cyclosporin A (CsA) and buthionine sulfoximine (BSO) alone as well as to cisplatinum, whereas the L0 and ACHN cells, also slowly growing fibroblasts and non-proliferating lymphocytes, were considerably less sensitive. Compared with AML cells and lymphocytes, CLL cells were more sensitive to Ver and CsA alone. The cytotoxicity of Vcr was significantly increased in the Vcr-resistant ACHN and LI00, but also in sensitive L0 cells by Ver and CsA, with smaller effects on Dox and Vp-16 toxicity. Fibroblasts and lymphocytes were generally resistant to the cytotoxic agents and RM addition had only minor effects. CLL cells were more sensitive to Dox and Vcr as compared with normal lymphocytes, with potentiation of the Vcr effect by Ver and CsA. The Vcr effect in non-proliferating Vcr-resistant cells from a malignant schwannoma was potentiated by Ver and CsA, which had no effect in cells from a kidney adenocarcinoma. We conclude that cytotoxicity of RMs alone is not dependent on the proliferation rate of tumor cells and that potentiation of cytotoxic drugs by RMs may be selective for tumor cells irrespective of their initial level and mode of drug resistance.

Use of verapamil to enhance the antiproliferative activity of BCNU in human glioma cells: an in vitro and in vivo study

The authors have evaluated the antiproliferative activity of verapamil, alone or in combination with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in brain-tumor cells. These effects were studied in vitro using four human glioma cell lines and in vivo using glioblastoma multiforme cells transplanted to athymic nude mice. The results showed that verapamil when used alone produced inhibition of tumor growth; however, when verapamil was used in combination with BCNU (in vitro), significant dose-dependent suppression of proliferation occurred in all four cell lines. The in vivo results were far more dramatic. Mice treated with BCNU (25 mg/kg) plus verapamil (50 mg/kg) achieved a 200-fold decrease in tumor growth with a greater than 80% regression in tumor size. Complete cures were achieved in 80% of the mice observed for at least 50 days following the completion of therapy. These findings support the use of verapamil in overcoming drug resistance in malignant brain tumors.

Addition of verapamil and tamoxifen to the initial chemotherapy of small cell lung cancer. A phase I/II study

Based on experimental observations that verapamil and tamoxifen reverse multiple drug resistance, the authors investigated the feasibility of combining both agents with the initial chemotherapy of extensive small cell lung cancer. Overall, in a consecutive series of 58 patients the most important toxicity was myelosuppression, and there was a 24% rate of severe infections. Therapeutic results included 24% complete and 34% partial response rates, median time to disease progression of 32 weeks, and median survival of 46 weeks. In three consecutive cohorts of patients the dose of either tamoxifen or verapamil were escalated by 25% and 33%, respectively. The cohort of patients receiving verapamil 360 mg/day and tamoxifen 100 mg/day (level 2) had slightly more toxicity but also more responses than the other groups. Therefore, the authors recommend that these doses be used in controlled trials to confirm the promising results of their study.

Toxicity of novel anthracycline derivatives towards normal myeloid bone marrow progenitor cells (CFU-GM) is not increased by verapamil

Drug-induced myelotoxicity is usually the dose-limiting factor of treatment of malignant tumors with cytostatic drugs. Suppression of in vitro myelopoiesis (CFU-GM) by cytostatics may be a suitable model reflecting the in vivo situation. Thus the inhibitory effects of the anthracyclines doxorubicin, theprubicin, idarubicin and cytorhodin S on CFU-GM were compared. Normal human bone marrow cells were incubated with these drugs for one hour and alternatively, for the whole culture period. For each substance and each incubation time a dose-response curve was established and the D50 determined. As certain calcium antagonists can increase the toxicity of some cytostatic drugs in various tumor models, the effect of the addition of verapamil (2 microM) was also investigated. It could be shown, that the myelotoxicity on CFU-GM of the drugs mentioned above was not increased after short-term or permanent exposure to this calcium antagonist.

Cyclobut-A and cyclobut-G, carbocyclic oxetanocin analogs that inhibit the replication of human immunodeficiency virus in T cells and monocytes and macrophages in vitro

Two newly synthesized carbocyclic oxetanocin analogs, (+/-)-9-[(1 beta,2 alpha,3 beta)-2,3-bis(hydroxymethyl)-1-cyclobutyl]adenine (cyclobut-A) and (+/-)-9-[(1 beta,2 alpha,3 beta)-2,3-bis(hydroxymethyl)-1-cyclobutyl]guanine (cyclobut-G) were tested for activity against the infectivity of human immunodeficiency virus (HIV) in vitro. A number of other carbocyclic oxetanocin analogs failed to exert good antiretroviral effects. Both cyclobut-A and cyclobut-G protected CD4+ ATH8 cells against the infectivity and cytopathic effect of HIV type 1 (HIV-1) and suppressed proviral DNA synthesis in ATH8 cells exposed to HIV-1 in vitro at concentrations of 50 to 100 microM. These compounds also inhibited the in vitro infectivity of another human pathogenic retrovirus, HIV-2. Furthermore, both compounds completely suppressed the replication of a monocytotropic strain of HIV-1 in monocytes and macrophages at concentrations as low as 0.5 microM, as assessed by inhibition of HIV-1 p24 gag protein production. We also found that 2'-deoxyguanosine readily reversed the antiretroviral activity of cyclobut-G in our system, whereas the activity of cyclobut-A was hardly reversed by 2'-deoxyadenosine or 2'-deoxycytidine. We noted, however, that these compounds inhibited the proliferation of peripheral blood mononuclear cells at concentrations of greater than or equal to 100 microM in vitro. Although both cyclobut-A and cyclobut-G appear to have a certain level of in vitro toxicity, our observations may have theoretical and clinical implications in understanding the structure-activity relationships of antiretroviral agents active against HIV.

In vitro inhibition of the infectivity and replication of human immunodeficiency virus type 1 by combination of antiretroviral 2',3'-dideoxynucleosides and virus-binding inhibitors

We tested the in vitro inhibitory activities of three 2',3'-dideoxynucleosides and two inhibitors of viral binding in combinations against the infectivity and cytopathic effect of human immunodeficiency virus type 1. 3'-Azido-2',3'-dideoxythymidine, 2',3'-dideoxyinosine, or 2',3'-dideoxycytidine, combined with recombinant soluble CD4 (sCD4), brought about synergistic antiretroviral activity without toxicity at clinically achievable concentrations. Combinations of 2',3'-dideoxynucleosides plus dextran sulfate exerted similar synergistic antiviral effects without concomitant increases in toxicities. When sCD4 and dextran sulfate were combined, apparent antagonism was observed. We confirmed that no combination of sCD4 plus 3'-azido-2',3'-dideoxythymidine, 2',3'-dideoxyinosine, or 2',3'-dideoxycytidine significantly increased the inhibitory effect on colony formation of human myeloid-monocytic bone marrow cells in vitro at the concentrations used in this study. These data might have clinical relevance for the treatment of patients infected with human immunodeficiency virus.

Effects of calcium antagonists on anti-cancer drug toxicity to haematopoietic progenitor cells in normal human bone marrow

Calcium (Ca) antagonists are promising candidates to enhance the drug sensitivity of resistant tumor cells. However, it is not yet clear whether these agents increase anti-cancer drug toxicity to normal human haematopoietic progenitor cells. In this paper, we examined the ability of Ca antagonists including verapamil, diltiazem, nicardipine, and clomipramine, to augment the toxicity of vincristine (VCR) and adriamycin (ADM) to normal bone marrow granulocyte-macrophage progenitor cells (CFU-GM) in vitro. When CFU-GM colonies were cultured for 10 days with VCR(10(-9) M) or ADM(10(-8) M) alone, the number of colonies was reduced to 73.7% +/- 13.0% (mean +/- S.D., n = 9) and 63.4% +/- 10.2% (n = 9) of those of the controls, respectively. The addition of Ca antagonists at the clinically achievable concentrations of 0.2 and 2 microM further enhanced the toxicity of these drugs; verapamil even at a lower concentration reduced the colony number to 36.8% +/- 8.9% (n = 4) for VCR and 32.8% +/- 9.5% (n = 4) for ADM. Nicardipine also reduced CFU-GM to a lesser degree in co-culture with ADM. Ca antagonists alone did not suppress CFU-GM. No enhancement of toxicity was mediated by Ca antagonists at lower VCR or ADM concentrations (10(-10) M or less). In addition, after a short-term (24 h) co-incubation of VCR or ADM with Ca antagonists, no significant enhancement of toxicity was observed. These results indicated that the combination of VCR or ADM with Ca antagonists may enhance toxicity to normal human CFU-GM under certain circumstances.

Chronic calcium antagonist use in carcinoma of the lung and colon: a retrospective cohort observational study

Detailed records were maintained prospectively of all medications taken by 719 patients with advanced carcinoma of the lung or colon. Of this total, a cohort of 19 patients was identified who had ingested incidentally either nifedipine, diltiazem, verapamil, or trifluoperazine in standard therapeutic doses for a minimum of one month and a mean of 5.8 months and median of three months. Treatment with these calcium antagonists was well tolerated and, upon comparison with otherwise comparable patients who did not ingest a calcium antagonist, appeared to be associated with certain favorable outcomes, including delayed tumor progression and prolonged survival. These preliminary findings suggest that beneficial effects of such drugs observed with chronic treatment in experimental animal tumor models may occur in human disease and that definitive prospective, randomized, clinical trials of calcium antagonists administered continuously in ordinary therapeutic doses are both feasible and justified.

Dipyridamole potentiates the inhibition by 3'-azido-3'-deoxythymidine and other dideoxynucleosides of human immunodeficiency virus replication in monocyte-macrophages

Dipyridamole (DPM) is commonly used as a coronary vasodilator and inhibitor of platelet aggregation in the treatment of cardiovascular diseases. We report here that DPM potentiates the inhibitory effects of 3'-azido-3'-deoxythymidine (AZT) and 2',3'-dideoxycytidine against human immunodeficiency virus type 1 (HIV-1) in human monocyte-macrophages. At the same concentrations, DPM does not potentiate the toxic effects of AZT on these cells or on human bone marrow (granulocyte-monocyte) progenitor cells. Since monocyte-macrophage lineage cells appear to be the major reservoir for HIV-1 in vivo, these findings suggest the possibility of using DPM or its analogues in combination chemotherapy of HIV infections.

Diltiazem does not increase the in vivo sensitivity of murine hemopoietic progenitor cells to doxorubicin

The effect of doxorubicin and the calcium antagonist, diltiazem, on murine hemopoietic progenitor cells was studied in vivo. Dose-survival curves of murine bone marrow colony forming units (CFU)--spleen and granulocyte macrophage--were determined by in vivo and in vitro methods in DBA/2NCr/BR mice treated with doxorubicin alone or by simultaneous administration of doxorubicin (DX) and diltiazem (DTZ). Time response of bone marrow hematopoietic progenitor cells (HPC) was followed in mice similarly treated. Combination of DTZ with DX did not change the toxic effect of the latter on hemopoietic cells, either in the dose-survival model or in the time-related experiment.