Results of Pharmaceutical Preclinical Studies

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1. Five drugs have been identified as being potentially useful to develop a new antidepressant which would act as a triple reuptake inhibitor, i.e. selectively inhibit serotonin, noradrenaline and dopamine reuptake but be devoid of the anticholinergic and antihistaminergic adverse effects associated with tricyclic antidepressants (such as imipramine).

Table 1 summarises the results obtained for the affinities of a representative chemical from each drug class for transporters and certain receptors:

In vitro profile for a series of test compounds

Drug                SERT                NART               DART               Muscarinic      H₁

M&B 28,574    1.3+0.5            2.0+0.2            1.9+0.4            785.1+47.6      856.5+77.5

M&B 25,678    158.2+22.2      471.3+58.1      450.2+65.7      15.7+1.3          8.7+0.7

M&B 22,843    2.3+0.5            2.0+0.2            1.9+0.4            >1000              5.3+0.5

M&B 29,595    1.6+0.4            3.8+0.5            17.8+0.7          32.3+3.3          >1000

M&B 28,772    15.5+1.5          241.2+52.1      >1000              >1000              >1000

Imipramine     3.5+0.3            2.7+0.3            12.2.+0.6         23.8+5.3          25.4+2.5

Data are expressed as mean + S.E.M. Ki (nM) of at least 3 separate experiments, each performed in triplicate. SERT: serotonin reuptake transporter; NART: noradrenaline reuptake transporter; DART: dopamine reuptake transporter.

From this information, which drug would you select for further investigation? Give reasons for your answer.

2. There is a continual need to develop better drugs for the treatment of schizophrenia (i.e. antipsychotics) that would be potent and that are effective when administered orally. Figure 1 illustrates the effects of the test compound when administered intravenously to rats on the hyperactive response induced by amphetamine. Comment on the results, including in your answer the rationale for inclusion of risperidone, as well as what you would consider the next stage in developing the profile for this compound.

The test compound or risperidone were administered intraveneously 60 min prior to a subcutaneous injection of amphetamine (2 mg/kg). Home cage locomotor activity was subsequently measured for a period of 30 min. Results are expressed as mean + S.E.M. Distance Moved. * = p<0.05 vs Saline control; + = p<0.05 vs Amphetamine control (n=8, Student Newman Keuls test).

3. A new antiepileptic compound (M&B 45,298) was evaluated preclinically for efficacy and safety. The animal model used was the pentylenetetrazole seizure test. Pentylenetetrazole is a compound that induces seizures in mice, and the ability of drugs to block these seizures is indicative of antiepileptic potential.

The following results were obtained in the tests (Table 2), for M&B 45,298 for both the intravenous and oral routes of administration, as well as its ability to produce excessive sedation, a toxic consequence. Phenytoin is a currently marketed antiepileptic drug. Comment on the results.

Antiepileptic activity of M&B 45,298

                                    Pent. Test                             Sedation

                                  (ED50, mg/kg)                   (TD50, mg/kg)

Evaluation of Antidepressant Candidates

Intravenous

M&B 45,298                125                                          520

Phenytoin                    263                                          398

Oral

M&B 45,298                120                                          538

Phenytoin                    646                                          784

Results are expressed as the ED₅₀ or TD₅₀ for each drug, and for each route of administration.

4. Falindoxole is a novel antimalarial compound that was evaluate in the Ames bacterial mutagenicity test, and in the mouse micronucleus test. The results are presented in Tables 3 and 4

The effects of falindoxole in the Ames test

Compound                 -S9                   +S9 

Falindoxole                  >1000              200

MNNG                         20                    20

Results are expressed as the lowest detectable mutagenic response (mg/plate), in the presence or absence of the S9 fraction. MNNG (N-methyl-N'-nitro-N-nitrosoguanidine) is a positive control.

The effect of falindoxole in the mouse micronucleus test

Group                         % micronuclei

Vehicle                         0.8 + 0.08%

500 mg/kg                   1.0 + 0.12%

1000 mg/kg                 0.9 + 0.11%

2000 mg/kg                 1.1 + 0.10%

40 mg/kg CPA             21.7 + 2.3^*%

Mice received to oral doses of either vehicle or the test drugs, 24 h apart. Following a further 24 h, bone marrow cells were harvested. Results are expressed as mean (+ S.E.M.) % micronuclei observed in bone marrow cells. CPA is cyclophosphamide, a positive control. * = p<0.05 vs. Vehicle.

Comment on the results and whether you would consider developing falindoxole further

g seizures in humans, it has the considerable drawback of being a known teratogenic compound. The present investigation evaluated a series of structurally related compounds for their therapeutic and teratogenic potential in mice. The results are summarised in Table 5

The efficacy and teratogenic potential of a series of valproate analogues in mice

Group             ED₅₀                             Resorption                 Malformations

                        (mg/kg)                       (%)                               (%)

Control            NA                               4%                               0%

VPA                  150                              4%                               82%

A                      >500                            4%                               0%

B                      50                                25%*                           0%

C                      125                              22%*                           7%*

D                      50                                3%                               0%

The ED₅₀ was defined as the dose that would block evoked seizures by 50%. In the teratogenicity study, all mice received 500 mg/kg of the test compound during gestation, and sacrificed immediately prior to littering. Resorption is the percentage of embryos resorbed, whilst malformations indicates the percentage of live foetuses possessing some form of malformation.

Comment on the results and whether you would consider putting forward any of the compounds for further development.

6. An antihypertensive drug that would confer an improved survival rate in patients with heart failure would be a very desirable feature, as well as one that would possess less adverse effects. M&B 52,834 is a new antihypertensive that was compared to 2 currently marketed  ntihypertensive drugs, namely losartan and captopril.

A double-blind, randomised trial involved allocation to one of three groups and followed for 1.5 years, and the safety and tolerability were examined, namely the total number of patients who died, and the number of patients that withdrew due to adverse effects. The following results were obtained:

Comparison of M&B 52,834 with losartan and captopril on longterm safety and tolerability in patients with heart failure

Group                         Total mortality          Patient withdrawals

M&B 52,834                286 (18.0%)                 125 (7.9%)

(n = 1585)

Losartan                      292 (18.3%)                 180 (11.3%)

(n = 1596)

Captopril                     298 (18.6%)                 260 (16.3%)

(n = 1598)

p value                        0.75                             0.001

Results are expressed as the number of patients (with percentages in brackets) for total mortality and the proportion of patients that withdrew from the study due to adverse effects. A significant effect between groups occurs where the p value is <0.05.

Comment on the results, and the experimental group composition. Do you think that M&B 52,834 is superior to the other drugs?