WO2011045554A1

WO2011045554A1 - Auto-injector system for variable dose

Info

Publication number: WO2011045554A1
Application number: PCT/GB2010/001770
Authority: WO
Inventors: Douglas Ivan Jennings
Original assignee: Pa Knowledge Limited
Priority date: 2009-10-12
Filing date: 2010-09-21
Publication date: 2011-04-21
Also published as: GB0917855D0; GB2474308A

Abstract

An auto-injector system for use with a plurality of syringes containing different doses of fluid to be injected into a human or animal body, the auto-injector system comprising: a first housing comprising a first connector, and a second housing comprising a second connector, the first and second connectors being configured to interact with each other such that the first housing is connectable to said second housing; a syringe receiving portion for holding one of said plurality of syringes; a push rod and an actuation mechanism, both mounted in said second housing and configured such that, when a syringe is held within said syringe receiving portion and said first and second housings are connected together, actuation of said actuation mechanism causes said push rod to move axially relative to said second housing from a pre-actuation position to an axially advanced position, movement of said push rod from said pre-actuation position to said axially advanced position being effective to force a seal member sealing fluid in said syringe to move axially towards the needle end of the syringe, thereby expelling said fluid from said syringe through the needle thereof; and a spacer configured to cooperate with said first and second connectors such that the axial separation of said first and second housings is greater when the first and second housings are connected together using said spacer than when said first and second housings are connected together without using said spacer.

Description

AUTO-INJECTOR SYSTEM FOR VARIABLE DOSE

The present invention relates to an auto-injector system, and in particular to an auto-injector system for injecting a plurality of different dose volumes.

The general concept of an auto-injector system is known in the prior art. Such systems comprise means for receiving a syringe containing a particular dose, and for automatically injecting the dose into a human or animal subject. Typically, a user will place the device into contact with the area to be injected and trigger an actuation mechanism (for example, by depressing a button or switch). The actuation mechanism will drive a push rod to advance a syringe towards a subject to be injected (to pierce the skin) and then to advance a seal member or stopper within the syringe to inject the dose. WO 2005/115513 Al and WO 2005/115510 Al, for example, disclose auto-injector systems according to the prior art.

Typical prior art auto-injector systems operate by substantially emptying the syringe of fluid on each actuation of the auto-injector. Different doses can be achieved in such systems by changing the amount of fluid that is present in the syringe prior to actuation. In practice, this will be done by inserting syringes containing different amounts of fluid.

The variation in dose volume causes a corresponding variation in the position of the seal member prior to actuation: for larger dose volumes, the seal member will be positioned relatively far from the needle; and for smaller dose volumes, the seal member will be positioned closer to the needle.

The actuation mechanism may be arranged so that the push rod is in the same position prior to actuation for all doses. Where the seal member position varies, therefore, the separation between the end of the push rod and the seal member will also vary: for the largest doses envisaged, the push rod can be in contact with the seal member prior to actuation (or very close thereto), but for smaller doses there will be a gap between the push rod and the seal member prior to actuation.

If the gap is left unfilled, it may be difficult to ensure consistent operation between different doses. This is because the position of the push rod relative to the actuation mechanism (e.g. a spring and its anchoring point) when it first comes into contact with the seal member will vary according to the dose to be injected, which may affect the force applied by the actuation mechanism as the seal member is advanced. Furthermore, while the push rod advances through the gap, there is relatively little resistance to counteract the force applied by the actuation mechanism. If no counter measures are taken, this could lead to substantial acceleration of the push rod prior to impact with the seal member. Increased impact forces may again have a negative effect on the consistency of operation for different doses and may also reduce the reliability and/or longevity of the device by subjecting components to increased stress. As a solution to some of the problems mentioned above, a spacer may be provided between the push rod and the seal member, the length of the spacer being adapted according to the dose to be injected so that there is no gap behind the seal member (i.e. the spacer effectively fills the gap that would have existed between the push rod and the seal member). This arrangement is advantageous in that the force applied to the seal member, at least for the initial portion of the push rod movement (for example, the portion equivalent to the maximal displacement for the smallest dose envisaged), does not vary as a function of the size of dose being dispensed. There are also no impact forces associated with the push rod traversing a gap before coming into contact with the seal member. However, such an arrangement is difficult to assemble: a different set of internal components have to be assembled for each different dose. Because the components that need to be changed are internal, they are relatively small and can be difficult to handle, particularly for certain patient groups, such as the elderly for example. Where the components are changed at a factory, manufacturing costs will increase. Furthermore, the device as a whole has to be long enough to accommodate the maximum envisaged dose, even when smaller doses are to be dispensed, which limits compactness.

It is an object of the present invention at least partially to solve one or more of the above- mentioned problems of the prior art.

According to an aspect of the invention, there is provided an auto-injector system for use with a plurality of syringes containing different doses of fluid to be injected into a human or animal body, the auto-injector system comprising: a first housing comprising a first connector, and a second housing comprising a second connector, the first and second connectors being configured to interact with each other such that the first housing is connectable to said second housing; a syringe receiving portion for holding one of said plurality of syringes; a push rod and an actuation mechanism, both mounted in said second housing and configured such that, when a syringe is held within said syringe receiving portion and said first and second housings are connected together, actuation of said actuation mechanism causes said push rod to move axially relative to said second housing from a pre-actuation position to an axially advanced position, movement of said push rod from said pre-actuation position to said axially advanced position being effective to force a seal member sealing fluid in said syringe to move axially towards the needle end of the syringe, thereby expelling said fluid from said syringe through the needle thereof; and a spacer configured to cooperate with said first and second connectors such that the axial separation of said first and second housings is greater when the first and second housings are connected together using said spacer than when said first and second housings are connected together without using said spacer.

Thus, the system adapts to different doses by varying the separation of the two housings using a spacer. This approach can avoid significant gaps between the leading face of the push rod and the seal member, which would otherwise lead to large impact forces, without requiring modification of small internal components, or the addition of internal spacers between the push rod and the seal member, for example. In addition, the overall size of the device can vary as a function of the dose. When the dose is small, the device can be made correspondingly compact.

Furthermore, the fact that the same first and second housing structures can be used for a variety of different doses (only the spacer varies between different doses) enables production costs to be reduced, for example through economies of scale in manufacturing and/or improved assembly logistics.

The syringe receiving portion can be attached to either the first or the second housing. When attached to the first housing, the syringe receiving portion can be configured to attach to the same portion of the syringe regardless of the dose that the syringe contains. This facilitates manufacture, both from the point of view of constructing the syringe receiving portion and from the point of view of mounting the syringe into the syringe receiving portion. When the syringe receiving portion is attached to the second housing, along with the push rod and actuation mechanism, the construction of the first housing is greatly simplified.

A plurality of spacers can be provided, each corresponding to a particular one of the plurality of syringes containing different doses of fluid to be injected.

The spacer may be configured so that the axial separation between a leading edge of said spacer and a trailing edge of said spacer is adjustable. In this way, it is possible to provide a single spacer that can be used with syringes containing a plurality of different doses.

The spacer can be provided as a separate element, connected neither to the first nor the second housing. Alternatively, the spacer can be connected to one of the housings and configured to be switchable between: a first state, in which the spacer does not cooperate with the first and second connectors when said first and second housings are connected together, the axial separation of said syringe receiving portion and the leading face of said push rod when in said pre-actuation position thereby remaining unchanged by said spacer; and a second state, in which the spacer does cooperate with the first and second connectors when said first and second housings are connected together, the axial separation of said syringe receiving portion and the leading face of said push rod when in said pre-actuation position thereby being changed by said spacer. In this way, fewer separate elements are required, which may facilitate assembly and/or reliable storage, for example. There is less chance of misplacing the spacer if it is connected to one of the housings.

According to a further aspect of the invention, there is provided a method of using an auto- injector comprising: inserting one of a plurality of syringes containing different doses of fluid to be injected into a human or animal body into a syringe receiving portion; connecting a second housing to said first housing, said second housing having a push rod and an actuation mechanism mounted therein; actuating said actuation mechanism to cause said push rod to move axially relative to said second housing from a pre-actuation position to an axially advanced position, movement of said push rod from said pre-actuation position to said axially advanced position being effective to force a seal member sealing fluid in said syringe to move axially to the needle end of the syringe, thereby expelling said fluid from said syringe through the needle thereof, wherein: said connecting step comprises inserting a spacer in a space between corresponding abutment surfaces of said first and second housings to increase the axial separation between said first and second housings.

The invention will be more clearly understood from the following description, given by way of example only, with reference to the accompanying drawings, in which:

Figure 1 A is a perspective view of an assembled auto-injector system with spacer according to an embodiment of the invention;

Figure IB is an exploded perspective view of the assembled auto-injector system with spacer shown in Figure 1 A;

Figure 1C is a sectional end view of the assembled auto-injector system with spacer shown in Figure 1 A;

Figure ID is a sectional side view of the assembled auto-injector system with spacer shown in Figure 1A;

Figure IE is a sectional top view of the assembled auto-injector system with spacer shown in Figure 1A;

Figure 2 A is a perspective view of an auto-injector system assembled without a spacer; Figure 2B is an exploded perspective view of the assembled auto-injector system shown in Figure 2A;

Figure 2C is a sectional end view of the assembled auto-injector system shown in Figure 2 A; Figure 2D is a sectional side view of the assembled auto-injector system shown in Figure

2A;

Figure 2E is a sectional top view of the assembled auto-injector system shown in Figure 2A;

Figures lA to IE show an example auto-injector system 100 assembled using a spacer 101 to allow delivery of a relatively large dose of fluid. Figures 2 A to 2E show the auto-injector system 100 assembled without the spacer 101 so as to allow delivery of a relatively small dose of fluid. Figures lAand 2A are perspective views, Figures IB and 2B are exploded perspective views, Figures 1C and 2C are sectional end views, Figures ID and 2D are sectional side views and Figure IE and 2E are sectional end views.

The auto-injector system 100 comprises a first housing 102 and a second housing 104. In the embodiment shown, a syringe receiving portion 106 is connected to the first housing 102 for receiving and holding a syringe 108A/108B containing a pre-metered dose of fluid. The syringe 108A/108B comprises a reservoir 112, a needle 114 and a seal member 116 for sealing the fluid to be injected within the reservoir 112. The syringe 108 A that is present in the arrangement of Figures 1 Ato IE contains more fluid (i.e. larger dose) than the syringe 108B that is present in the arrangement of Figures 2A to 2E.

A removable cap assembly 118 may be provided for protecting the needle 114 prior to and/or after use.

A push rod 120 and actuation mechanism are attached to the second housing 104. In the embodiment shown, the actuation mechanism comprises a spring system 122 configured to engage with a rear end of the push rod 120 and apply a force axially in the direction of the needle end of the syringe 108A/108B, and a release button 124. In the configurations shown in Figures lA to 2E, the push rod 120 is held in a pre-actuation state, axially distal relative to the needle end of the syringe 108A/108B against the force of the spring system 122. The push rod 120 is prevented from moving towards the needle end of the syringe 108A/108B by a restraining member. Depression of the release button 124 disengages the restraining member and allows the push rod 120 to advance axially in the direction of the needle end of the syringe 108A/108B under the force from the spring system 122.

The system is configured such that, shortly after actuation, a leading face 126 of the push rod 120 will come into contact with the seal member 116 of the syringe 108A/108B. The force thus applied to the syringe 108A/108B is effective first to cause the syringe 108A/108B to advance axially and puncture the skin to be injected. This axial advancement of the syringe 108A/108B continues until a syringe abutment member 128 comes into contact with a corresponding abutment member 130 on the first housing 102, which prevents further movement. From this point, the force applied to the push rod 120 by the spring system 122 causes axial advancement of the seal member 116 relative to the syringe reservoir 112, forcing fluid to be expelled from the reservoir 112 through the needle 114.

The expulsion of fluid from the reservoir 112 will continue until the push rod 120 has advanced to the end of the reservoir 112 or to a pre-detenriined position within the reservoir 112.

A syringe spring system 110 may be provided for retracting the syringe 108A/108B once the injection process has completed. In the example shown, the syringe spring system 110 is configured to apply a retracting force to the syringe 108A/108B that is much smaller than the force provided by the spring system 122 of the actuation mechanism, which acts in the opposite sense. Thus, while the spring system 122 is engaged with the push rod 120, the spring system 122 forces dominate the opposing forces applied by the syringe spring system 110. However, in the present example means are provided for disengaging the spring system 122 from the push rod 120 when the injection process has been completed (i.e. when the push rod 120 has advanced as far as desired within the syringe reservoir 112). In particular, in the present embodiment, ramps 136 provided to a rear portion of the push rod 120 are configured to disengage spring system hooks 132, which connect the push rod 120 to the spring system 122, when the ramps 136 are pushed radially inwards by engagement, driven by the axial movement of the push rod 120, with a tapered opening 134 provided in the syringe receiving portion 106. Thus, at the end of the injection process, the syringe spring system 110 takes over from the spring system 122 of the actuation mechanism and causes the syringe 108A/108B to be axially retracted as required.

In the example shown, the auto-injector 100 is configured such that the position of the syringe needle 114 relative to the first housing 102 is always the same prior to actuation, regardless of the dose contained in the syringe 108A/108B. The position of the seal member 116 relative to the first housing 102 prior to actuation will therefore depend on the dose to be dispensed. For larger doses, the seal member 116 will be further away from the needle 114 than for smaller doses. This can easily be seen by comparing the position of the seal member 116 in Figures ID and IE with the position of the seal member 116 in Figures 2D and 2E. Prior to actuation, it is preferable for the leading face 126 of the push rod to be in close proximity to the seal member 116 but without being in contact, for example at a distance less than 10 percent of the axial depth of fluid in the syringe 108, more preferably less than 5 percent, more preferably still within 1 percent. The leading face 126 of the push rod 120 may also be arranged to be in contact with the seal member 116. Limiting the size of the gap helps to reduce the size of the impact forces arising when the push rod 120 is driven into contact with the seal member 116. If the gap is larger, there is more scope for the push rod 120 to be accelerated to high speeds before corning into contact with the seal member 116, which means larger impact forces. Large impact forces will in general have a negative effect on reliability and/or longevity, or increase production cost and/or device weight (if more robust components have to be used to compensate, for example).

If the position of the seal member 116 relative to the first housing 102 varies as a function of dose while the leading face of the push rod 120 has to be maintained close to the seal member 116, the position of the leading face 126 of the push rod 120 relative to the first housing 102 prior to actuation also has to vary as a function of dose.

In the arrangement shown in Figures 1 A to IE, this is achieved by introducing a spacer 101 that is configured to fit between the first housing 102 and the second housing 104 and define the separation therebetween. As mentioned above, in the pre-actuation state, there is a fixed (i.e. dose independent) relationship between the position of the needle of the syringe 108A/108B and the first housing 102. There is also a fixed (i.e. dose independent) relationship between the position of the push rod 120 and the second housing 104. Thus, by varying the separation between the first and second housings 102/104 using the spacer 101, it is possible to vary the position of the leading face 126 of the push rod 120 so that it is always in the appropriate position just behind the seal member 116 prior to actuation, regardless of the dose. For larger doses, the spacer 101 needs to be larger (i.e. it needs to be such as to impose a larger relative separation between the first and second housings 102/104) than for smaller doses. For the smallest dose envisaged, no spacer 101 is required; this is the situation in the arrangement shown in Figures 2A to 2E.

The first and second housings 102/104 may respectively comprise first and second connectors, which can engage with each other to connect the first housing 102 to the second housing 104. The spacer 101 may comprise identical connectors at either end, to allow the spacer 101 to be connected between the first and second housings 102/104. In other words, the spacer 101 may be configured to connect to the first housing 102 using a connector similar or identical to the second connector of the second housing 104, and be configured to connect to the second housing 104 using a connector similar or identical to the first connector of the first housing 102.

Alternatively, the first and second connectors may be configured so that the first and second housings 102/104 can be connected to each other over a range of longitudinal separations, for example using an extended axial thread. In this case, the spacer 101 may be configured simply to block relative movement of the first and second housings 102/104 for all separations below a certain threshold, defined for example by the length of the spacer 101. For example, the spacer may act simply as a wedge between the first and second housings 102/104. This arrangement avoids the need for the spacer to have its own connectors and simplifies construction of the spacer. A locking mechanism may be provided for locking together the sandwiched first housing 102, spacer 101 and second housing 104 once assembled, to help prevent in advertent movement of the first housing 102 away from the second housing 104.

The first and second housings 102/104 may respectively comprise complementary first and second abutment surfaces 103/105 (see Figures IB and 2B, for example), which are such that when the first and second housings 102/104 are connecting together, there is a circumferentially continuous line of contact between the first and second housings 102/104 (see Figure 2A, for example). In this way, the exterior surface of the device 100 in the region of contact between the first and second housings 102/104 is substantially continuous and/or smooth, facilitating a favourable aesthetic impact, but also helping to ensure that the interior of the device is protected from the external environment and contamination.

The spacer 101 may be configured to achieve a similar effect: namely, that the exterior surface of the device 100 in the region of contact between the first housing 102 and the spacer 101, and between the spacer 101 and the second housing 104, is substantially continuous and/or smooth. This may be achieved, for example, by arranging for the spacer 101 to comprise abutment surfaces 107/109 (axially leading and trailing edges) that are complementary with the abutment surfaces 103/105 of the first and second housings 102/104. For example, a first spacer abutment surface 107, defined by an axially leadmg edge of the spacer 101, may be arranged to have the same form as the second abutment surface 105 (on the second housing 104), so as to be complementary to the first abutment surface 103 (on the first housing 102). Similarly, a second spacer abutment surface 109, defined by an axially trailing edge of the spacer 101 , may be arranged to have the same form as the first abutment surface 103 (on the first housing 102), so as to be complementary to the second abutment surface 105 (on the second housing 104).

The spacer 101 may be substantially cylindrically hollow. By the word "substantially", what is meant, for example, is that the shape of the hole through the spacer 101 may be predominantly in the form of a cylinder, but include features such as connectors that protrude radially inwards by a small amount (e.g. less than 10 percent of the cylinder radius) in order to achieve their function.

The exterior form of the spacer 101 may comprise a portion of a cylinder.

In the arrangement of Figures 1A to IE, the spacer 101 is of fixed length and is only suitable for use with syringes 108A containing one particular dose volume (or narrow range of dose volumes). Where it is envisaged that the auto-injector will be used with more than two different dose volumes, more than one type (for example, length) of spacer 101 may be provided, i.e. an additional spacer will in general be required for each additional dose volume envisaged.

Alternatively, a spacer may be provided that can be adjusted so as selectively to impose one of a plurality of different separations between the first and second housings 102/104, and thereby between the leading face 126 of the push rod 120 and the needle 114 of the syringe 108A/108B. For example, the spacer maybe telescopic, comprising a plurality of nested units of different radii which can move axially relative to each other in order to change the distance between an axially leading edge and an axially trailing edge of the spacer. Alternatively or additionally, the spacer may be adjustable in length in the manner of a concertina.

In the arrangement shown in Figures lAto IE, the spacer 101 is formed as a separate element and is not attached to either of the first and second housings 102/104 (or, indeed, any other part of the auto-injector 100), except when blocked between the first and second housings 102/104 during use. However, other arrangements are possible. For example, either or both of the first and second housings 102/104 may be provided with extension mechanisms which can be selectively extended in an axial direction to adjust the relative separation of the first and second housings 102/104 after they are connected together. The extension mechanisms may comprise spacer elements that have structures substantially the same as one or more of the various embodiments of the spacer discussed above (except for the fact that they are provided as part of the first and/or second housings 102/104 rather than as separate elements). For example, the spacer elements may be configured to extend telescopically from one or both of the first and second housings 102/104, or by means of a concertina arrangement.

A spacer element connected to the first or second housing 102/104 may be provided that is switchable between a first state in which the spacer does not engage with the first and second connectors and does not influence the separation between the first and second housings 102/104, and a second state in which the spacer is brought into engagement with the first and/or second connectors in order to adjust the relative separations of the first and second housings 102/104. For example, the spacer may be arranged so as to have an axial opening of adjustable shape and/or size, so as to be able to fit inside of the first or second housing 102/104 (when the spacer is made smaller) or outside of the first or second housing 102/104 (when the spacer is made larger). When the spacer is to be engaged so as to define a separation between the first and second housings 102/104, the spacer is moved axially into the gap between the first and second housings, and made to change shape so as to fit within the gap and block movement of the first housing 102 towards the second housing 104. If the spacer was previously stored inside of the first or second housing 102/104, this change of shape will involve some degree of radial expansion. Similarly, if the spacer was previously stored outside of the first or second housing 102/104, this change of shape will involve some degree of radial contraction. Preferably, the spacer will change shape in such a way as to conform with the outer profile of the first and second housings 102/104 in the region of connection between the spacer and the first and second housings 102/104.

In the arrangements discussed above, the syringe receiving portion 106 is connected to the first housing 102. However, it is also possible to provide a syringe receiving portion that is connected to the second housing 104 instead of the first housing 102. In this case, the syringe receiving portion would have to be configured so as to be capable of holding syringes containing different doses in different ways (e.g. gripping the syringes at different portions along their length), so that the push rod is brought into a suitable position just behind the seal member for each syringe. The extent to which the attached syringe will protrude from the second housing 104 will therefore vary, but this variation can be compensated by a spacer in the same way as in the above-described embodiments, such that when the first housing 102 is connected to the second housing 104 (using the spacer), the position of the needle of the syringe relative to the attached first housing 102 remains independent of dose.

The outer surface of the spacer 101 may be adapted to perform functions other than simply maintaining the required spacing between the first and second housings 102/104. For example, the outer surface of the spacer may comprise gripping means. This could be implemented by providing a rubberized surface, or other coating having a high coefficient of friction. Additionally or alternatively, texture could be applied to improve grip. This could be arranged to be equally effective in all directions or could be arranged to have a degree of directionality. For example, where increased grip in the axial direction is desired, which may help a user to hold the device in place prior to actuation, circumferentially orientated ribbing may be applied. Other structures such as a finger grip or flange could be provided.

Alternatively or additionally, the outer surface of the spacer 101 may be configured so as to provide a visual and/or textual indication to the user of the nature of the syringe that it contains (i.e. the dose that the device is configured to provide). The visual indication may consist of a distinctive colour, pattern or even wording, for example.

Claims

1. An auto-injector system for use with a plurality of syringes containing different doses of fluid to be injected into a human or animal body, the auto-injector system comprising:

a first housing comprising a first connector, and a second housing comprising a second connector, the first and second connectors being configured to interact with each other such that the first housing is connectable to said second housing;

a syringe receiving portion for holding one of said plurality of syringes;

a push rod and an actuation mechanism, both mounted in said second housing and configured such that, when a syringe is held within said syringe receiving portion and said first and second housings are connected together, actuation of said actuation mechanism causes said push rod to move axially relative to said second housing from a pre-actuation position to an axially advanced position, movement of said push rod from said pre-actuation position to said axially advanced position being effective to force a seal member sealing fluid in said syringe to move axially towards the needle end of the syringe, thereby expelling said fluid from said syringe through the needle thereof; and

a spacer configured to cooperate with said first and second connectors such that the axial separation of said first and second housings is greater when the first and second housings are connected together using said spacer than when said first and second housings are connected together without using said spacer.

2. An auto-injector according to claim 1, configured such that, for any one of said plurality of syringes containing different doses, when said first and second housings are connected together and the syringe is mounted in said syringe receiving portion, the needle of said syringe will be at a single pre-determined position relative to said first housing while said push rod is in said pre- actuation position, said pre-determined position being the same for each of said plurality of syringes.

3. An auto-injector system according to claim 2, wherein said syringe receiving portion is connected to said first housing and configured such that when any one of said plurality of syringes is held therein prior to connection of said second housing to said first housing, the needle of said syringe will be in said pre-determined position relative to said first housing.

4. An auto-injector system according to claim 2, wherein said syringe receiving portion is connected to said second housing and configured such that a syringe can be held therein in a plurality of different axial positions.

5. An auto-injector system according to any one of the preceding claims, wherein a plurality of spacers of different length are provided, each spacer being such that, when used with said first and second connectors to connect together said first and second housings, the axial separation between said first and second housings is different.

6. An auto-injector system according to any one of the preceding claims, wherein:

said first and second housings respectively comprise complementary first and second abutment surfaces that are configured to fit against each other when said first and second housings are connected together without said spacer in order to provide a substantially continuous surface in the region of the connection; and

said spacer has an axially leading edge of the same form as said second abutment surface and an axially trailing edge of the same form as said first abutment surface.

7. An auto-injector according to any one of the preceding claims, wherein said spacer is cylindrically hollow.

8. An auto-injector according to any one of the preceding claims, wherein said spacer is configured so that the axial separation between a leading edge of said spacer and a trailing edge of said spacer is adjustable.

9. An auto-injector according to any one of the preceding claims, wherein said spacer is connected to said first housing or said second housing and is configurable so as to be switchable between:

a first state, in which the spacer does not cooperate with the first and second connectors when said first and second housings are connected together, the axial separation of said syringe receiving portion and the leading face of said push rod when in said pre-actuation position thereby remaining unchanged by said spacer; and

a second state, in which the spacer does cooperate with the first and second connectors when said first and second housings are connected together, the axial separation of said syringe receiving portion and the leading face of said push rod when in said pre-actuation position thereby being changed by said spacer.

10. An auto-injector according to any one of the preceding claims, wherein an outer surface of said spacer comprises one or more of the following: a texture for enhancing grip, a structure for enhancing grip, a material for enhancing grip, a texture for indicating the nature of the particular syringe that is suitable for use with said spacer, and a patterning for indicating the nature of the particular syringe that is suitable for use with said spacer.

11. An auto-injector according to claim 10, wherein said structure for enhancing grip includes a finger grip or a flange.

12. An auto-injector set, comprising:

an auto-injector system according to any one of the preceding claims; and

one of said plurality of syringes containing an amount of fluid such that, when the syringe is held in said syringe receiving portion, and said first and second housings are connected together using said spacer, or a corresponding one of said spacers when dependent on claim 5, the leading face of said push rod when in said pre-actuation position is within a distance of less than 10 percent of the axial depth of said fluid in said syringe from said seal member.

13. An auto-injector set according to claim 12, comprising:

said plurality of syringes and a corresponding one or more spacers, said set being configured such that, when one of said syringes is held in said syringe receiving portion, and said first and second housings are connected together using a corresponding one of said plurality of spacers, or no spacer, as appropriate, the leading face of said push rod when in said pre-actuation position is within a distance of less than 10 percent of the axial depth of said fluid in said syringe from said seal member.

14. Amethod of using an auto-injector comprising:

inserting one of a plurality of syringes containing different doses of fluid to be injected into a human or animal body into a syringe receiving portion;

connecting a second housing to said first housing, said second housing having a push rod and an actuation mechanism mounted therein;

actuating said actuation mechanism to cause said push rod to move axially relative to said second housing from a pre-actuation position to an axially advanced position, movement of said push rod from said pre-actuation position to said axially advanced position being effective to force a seal member sealing fluid in said syringe to move axially towards the needle end of the syringe, thereby expelling said fluid from said syringe through the needle thereof, wherein: said connecting step comprises inserting a spacer in a space between corresponding abutment surfaces of said first and second housings to increase the axial separation between said first and second housings.

15. An auto-injector system constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

16. Amethod for using an auto-injector substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.